Just thought I might weigh in here, with my thoughts on the new codex, and how to counter with our own units.
Skarband, The Exiled One: Stronger Bloodthirster, Instrument of Chaos and Breath of Chaos (Winds of Chaos). He loses his wings, but gains Fleet.
Now, this guy is not too slow, but he's not quite as fast as a normal Bloodthirster. Unless he rolls really high for his Fleet move, he can't catch our suits or skimmers. For killing him, I would recommend three weapons; the plasma rifle, ion cannon and cyclic ion blaster. A PR+CIB combo will chew through his wounds, as any wounds you cause with the CIB will punch through the Iron Hide, and the PR will wound him reliably. Ion cannon can always out-range him, and has a high enough ROF to break the invul save. If you can get enough markerlights onto him, a Skyray will chip away wounds too (using seeker missiles). Don't get too close though, he has an AP2 flamer and is nasty in close-combat.
Ku'Gath, the Plaguefather: So he's basically a Great Unclean One, Breath of Chaos (same as Winds of Chaos), Aura of Decay and Cloud of Flies. However, there are two important rules which he has. The first is Necrotic Missiles; he chucks Nurglings at the enemy, dropping a pie plate that wounds anything on a 4+. The other rule is Nurgling Infestation; this allows Ku'Gath (provided he has been on the table for 1 turn) to summon a base of Nurglings that arrive via DS close to Ku'Gath.
Ku'Gath is a real problem for Tau under 4th Ed, due to FNP. However, with AP1/2 negating it in 5th Ed, it becomes easier. Massing PR, railgun solid shot and FB into him will chip away his wounds, but it will take concentrated fire to do so. Try hitting him with multiple Broadside teams and Helios suits at close-range. Pile on the AP1/2 and his invul save will break. He has six wounds though, so consider what else you could shoot, if he's not in range to shower you in Nurglings.
Fateweaver, Oracle of Tzeentch: A winged though inferior Daemon Prince. On the other hand, a superior invul save, can shoot two different powers at two different units (Master of Sorcery and We are Legion), and he has all the shooting powers. He has 1 unique rule; Oracle of Eternity. Friendly units near him can re-roll any and all saves. On the flipside, for every unsaved wound Fateweaver takes, he has to make a Leadership test or he is removed.
Against Fateweaver, MP and ion cannon are highly effective. Likewise, massed pulse firepower and SMS are also quite efficient. He'll absorb a lot of fire, but all it takes is 1 unsaved wound and a failed Ld test, and he runs. Force that test with massed fire; Ionheads, Deathrains, Firestorms etc. A FoF will cause 1-2 unsaved wounds (not including the Devilfish's firepower), cause another unsaved wound with support shooting and he's likely to either die or fall back.
Generic Greater Daemons (going in order of cost):
Great Unclean One: Slow, can only get Breath of Chaos to shoot with, but has FNP and a bunch of wounds. Treat him like Ku'Gath, get AP2/1 into him en-masse.
Keeper of Secrets: Slightly faster with Fleet, Gifts let her force a model to lose an Attack, Hit and Run, move them around (Pavane, like Lash of Submission) and have defensive+offensive grenades. She can get Daemonc Gaze, which is essentially a neutron burst cannon, so she can kill Crisis if you are unwary.
Much like Skarbrand, she isn't too fast. Unlike Skarbrand, she can't get an armour save, and relies heavily on the 4+ to keep her alive. Hit her with all your S6+ weapons, and she'll die in short order. Ion cannon, MP and plasma are all effective ways to get wounds on her. Also, should you have spare railgun, use them to get wounds on her.
Bloodthirster: God-like stats. Upgrades are a plasma pistol (which doesn't overheat), and a few close-combat bonuses.
All your AP1/2 shooting must go into this guy. Seeker missiles are also a good option; hit him with a Pathfinder or SMT, and then unload a Skyray onto him. It's imperative to eliminate the Bloodthirster as soon as possible; he is fast, tough, packs an accurate S7 shot for killing skimmers, and he'll chew through Kroot tarpits in no time. Even Farsight can't stand up to it.
Lord of Change: Probably the most balanced of them all, and while dangerous he isn't a close-combat powerhouse. The way he works in combat is through Soul Devourer, which forces you to take LD tests for unsaved wounds. So you can potentially force 3-4 tests in 1 round of combat, which is great for removing annoying characters. The rest of the upgrades are pretty straightforward; turning a model into Spawn, assault multi-melta, shooting two powers a turn, shooting at two different units. He comes with Daemonic Gaze standard.
Much like with Fateweaver, massing fire is how to kill him. Unlike Fateweaver however, pulse weapons are not effective, because of the T6. Ion cannon, missile pod, plasma...throw the book at this guy. He is dangerous for Tau, because he packs accurate move+shoot anti-tank and anti-infantry, sometimes in the same shooting phase (depending on Gifts). He'll also hurt in combat. Teamed with Fatewaver they will be quite a headache, able to soak up your S6+ shooting, while launching multi-melta into your tanks (while at the same time shooting up infantry with Breath or Gaze).
The Masque: She can shoot 3 different units with Pavane. Other than that, she's basically a slightly more powerful Daemonette. Can't join units, so when she comes down, hit her with pulse fire. Her invul save will break to it no problems.
Epidemius: A Plaguebearer with multiple wounds and attacks. He has Cloud of Flies (so offensive and defensive grenades), and his Tally of Pestilence. The only way it's going to trigger the more nasty effects is if the Daemon player takes a couple of Nurgle Princes with wings and hacks up a lot of people in combat; Plaguebearers and Nurglings simply won't kill enough. I would ignore him, he's not much of a threat.
The Blue Scribes: Multi-wound Horror with multiple attacks. Their gimmick is that they have every shooting power, and can shoot twice at different units. Their problem is that when shooting the second time, you have to roll a dice to see if they do what you want. 50% of the time they use the same power as before; 50% of the time, they do what you wanted. They are a two-wound Scout essentially (T4, 4+ save), hit them with enough pulse fire and they'll dissapear.
Skulltaker: More potent Bloodletter (even more wounds if on a Juggernaut or Chariot). His special rule is quite deadly; he Rends on 50% of his attacks, and any Rending hit inflicts instant death. He has Iron Hide, and will probably be on a Juggernaut (so he can keep IC status and join a unit of Bloodcrushers). He has a decent number of wounds, but if you can negate his Iron Hide he is just Daemon Prince essentially. One very useful unit is Vespid; backed up with a couple of markerlight hits, you can chip away at his wounds and bodyguard. You don't risk your Crisis getting in rapid-fire range, and while the Vespid won't survive the subsequent charge, you can always charge in Kroot after shooting up the unit, to buy you time to reposition.
Generic Heralds; As above. The only difference is that the named Heralds have more Gifts standard, while generic Heralds can only take 3 gifts. They can take chariots/mounts though, so the Nurgle/Tzeentch/Slannesh Heralds may be more mobile than the named Heralds, and in some cases have more wounds (Tzeentch and Slannesh).
Thats it for HQ. Don't forget, Daemon players can take 2 Heralds per HQ slot.
Fiends of Slannesh: Fast and deadly close-combat Beasts, so they'll be assaulting fast. Up to 6 in a squad. They are fairly expensive though, so you won't see many. Low save, so massed pulse fire and missile pod will kill them quickly. Firestorms can kill them, but you do risk getting caught in combat (due to their 12" charge), so hang back if in doubt.
Flamers of Tzeentch: Extremely shooty, they have Warpfire and Breath of Chaos. One can take assault multi-melta power. Also expensive, but very dangerous for Tau. The default tactic by Daemon players is to fire 3-strong units into clusters of enemies and flame away. The Flamer bomb can be highly effective, so minimise by spreading out your forces and denying DS with cheap bodies (like Kroot). Their standard shooting is burst cannon ROF+range, but AP4 bolter, so mechanised FW's are safest. The Devilfish hull will prevent the Breath of Chaos from reaching them, and you'll be able to make 4+ cover saves against their normal shooting. They are not particularly tough, Scout Marine stats mostly. You can tie them up with Kroot, but they are Jump Infantry, so you'll have to catch them after they arrive.
Bloodcrushers of Khorne: Nasty assault unit. They are expensive though (same as vanilla Terminator), so again you shouldn't be facing down many of these. Pump as much AP3- into them as possible, they have to die before reaching combat. They are slow (Infantry, no Fleet), so you should be able to gun them down before they reach you. Usually form a bodyguard for Skulltaker.
Beasts of Nurgle: Joke unit. Too slow to reach combat, randomised attacks. Pulse fire if you are bored.
Bloodletters: Similar statline to PAGK. One of them can buy Rending, another can get an Icon. They don't have Fleet, so hit them with FoF. Their Hellblades are unlikely to do any damage to the Devilfish, and their save will break. If you have a spare Railhead drop submunitions onto them as well. Charge Kroot in if desperate, as their power weapons are totally wasted. It's a pretty good trade, because they are over twice as expensive as Kroot. Hit them with a good FoF, then charge in Kroot to hold them in combat.
Daemonettes: Rending nastiness, but only GEQ. Pulse fire will hurt them, and with the nerf to Rending, you can FoF without too much fear. Submunitions will clean them up too.
Plaguebearers: Tarpit unit. Extremely slow, and terrible in combat. Daemon player will probably camp them on the objective, or use them to screen other units. I would ignore them, you won't have enough AP1/2 to kill squads. They'll soak up pulse fire and AP3+ no problems, especially in cover.
Horrors: Only have Warpfire. One can get assault multi-melta, and one Horror per army can be turned into the Changeling (a named character). Glamour is annoying, but if you have Shadowsun nearby your FW's can hopefully ignore it. Mass pulse fire into them, they are only Guardsman in carapace. Kroot will tarpit them in combat, and depending on how you roll you may even win combat.
Nurglings: Infantry speed, so they're easy to see and counter. Submuniton rounds should they threaten to tie up units in combat.
Flesh Hounds: Basically MeQ statline. Beasts, so watch out for their quick assault. Karnak can boost the unit. Unlike Bloodletters, no power weapons, although Karnak (and another member of the unit) can get Fury of Khorne (Rending). Just hit them with a FoF, they can't really hurt a Devilfish. Submunition rounds are also effective.
Seekers: Daemonettes on cavalry. FoF, or just hit them with massed pulse fire usin Firestorms.
Screamers of Tzeentch: Only purpose is to kill armour (they have meltabombs). Do not let them charge your vehicles; mass pulse fire them to death as a priority, they are jetbikes and therefore highly mobile. With the new rules about hitting non-WS in the rear armour, you don't want them anywhere near your skimmers.
Furies: Generic Daemon, you won't see them used. They're Jump Infantry, but they lack any real assault potential outside their MeQ statline. Much like Hounds, they'll die to a good FoF. They are somewhat effective as a tarpit unit (against Crisis etc), so kill them should they threaten to assault.
Soul Grinder: AV13 Defiler. A real problem for both vehicles and infantry, and it's nasty in close combat. Railguns, seeker missiles (into the rear armour), fusion Piranha. MP and ion cannon are only good if you can target the rear.
Daemon Prince: Most versatile and customizable unit. Kind of a cheaper, slightly worse version of the Greater Daemons, with all the upgrades available. If no Iron Hide, mass pulse fire; if it has Iron Hide (which invariably it does), use your AP3- firepower. They will always have wings (despite the expensive, 2/3rds of base cost)
Now, onto units I would take;
Her fusion blasters can take care of Soulgrinders, and put wounds on the Greater Daemons. Her Ld bubble is useful for ignoring things like Glamour and Pavane, as well as generally keeping things like Broadsides and static FW's from running due to casualties.
Shas'O, PR, CIB, SG, Stims, hw multi
He has a high ROF for killing infantry, and the AP1/2 for engaging Nurgle and Iron Hide. He makes a good companion to a Vespid team, and is reasonably survivable.
Tm L, BC, MP, MT, 2 x shield drones, BK
Shas'ui, BC, MP, MT
Relatively inexpensive, good range, and pumping out enough firepower to put a significant dent in infantry squads. They are tougher than a comparable Stealth team, and have the MP fire for engaging tougher targets (like Lords of Change). I would take 2-3 of these, they are very effective, flexible and make good support for a FoF.
Sha'vre, AFP, twin-MP, hw multi, shield drone
Twin-MP for putting wounds on GD's and Heralds, AFP for fragging Daemon Troops. A good auxilary to your Firestorm teams.
12 x FW's, pulse rifles
Warfish, SMS, TA, MT, DP
Expensive, but you can usually outmanovure the Troops and some of the Elites with this team. The DL isn't really neccessary, a 5+ cover save is usually better against Daemonic anti-tank shooting. You can always add Flechette Dischargers, but they are so-so IMO. Funny though, when a large squad of Bloodletters or Daemonettes tries to charge.
10 x Kroot
Take a couple of these squads, and spread them out to maximum coverage to prevent DS options. They will also be useful for slowing down the Daemonic assault, after you've hit them with your firepower.
Piranha, FB, TA, seeker missile
Cheap and disposable, it can be useful for blocking assaults from the Keeper of Secrets (GUO won't reach you, and the other two will just fly over), when you add the spread from the drones. Fusion blaster is mainly for taking out any Soulgrinders that appear, and the seeker missile for putting a wound on Iron Hide Daemons. I wouldn't take more that 2.
You can take Pathfinders here, but it's not essential. I don't recommend them, but if you really need the markerlights, go for it. They are horribly vulnerable to counter-assault though.
Railhead, BC, MT, DP, DL
Solid shot is mainly for putting wounds on the GD's and killing Soulgrinders, otherwise I would fire off the submunitions into infantry. BC's to put even more wounds into the target. With a 5+ cover save and the DL's, you have a pretty survivable tank.
Ionhead, BC, MT, DP, DL
Pretty cheap, and great for tackling Iron Hide units, as well as killing infantry.
BASS Tm L, twin-PR, hw mt, 2 x shield drones, BK
BASS Shas'ui, twin-PR
Given the assault nature of the Daemon codex, you are quite likely to enter 12" shooting range. Outside that range, the railguns will kill Soulgrinders and put wounds on the GD's. They can form a powerful anchor to your firebase. I would only take 1-2, you really need a Railhead or Ionhead for additional anti-infantry/GD firepower.
Skyray, BC, TA, MT, DP, DL
If you have a SMT or a Pathfinder squad, this can be a powerful anti-GD unit. A good seeker salvo will make it easier to finish off the beast with railgun and ion cannon. The BC have the higher ROF you'll need to put wounds on Princes and mop up infantry.
Tau should do pretty well at shooting up Daemons, and if you use Kroot wisely to tarpit survivors or the MC's, you can avoid assault. Anything with 12" movement or assault range should be a priority target, especially for your Crisis and skimmers. I didn't recommend them, but should you have spare points, gun drones can be useful for blocking assaults and mopping up units (everything is Fearless so Pinning won't happen at all). I would just use the free ones off Piranha (or Devilfish if you are trying to save points).
Deployment and LOS are going to be critical. Given the weird deployment of Daemons, you need to force anything that does deploy to come down right into your fire lanes. Only the Tzeentchian Daemons have real shooting ability (outside of the GD's and Princes), so all the others will be waiting for next turn to assault (after some repositioning with Run, under 5th Ed).
Strike hard, strike fast.