Well, I just bought a new chaos army + codex, and I have to say, they've lost some stuff.
Having said that, the icon of tzzentch gives every units that carries it +1 to invulnerable saves. If the unit doesn't have the invulnerable save, they get a 5+ one
My Tzeentch Sorcerer has a 4+ invul, plus its 2+ termi save.
Demonweapons are nasty, adding D6 power weapon attacks to a lord, plus bonuses for Marks. On a roll of a pone, model suffers an unsavable wound and doesn't attack.
Unmarked also grants +1 strength.
Khorne rolls +2D6 attacks. if either dice is a 1, see above.
Nurgle wounds on a 4+
Tzeentch is also a ranged weapon. Rng 24. S3 AP3 Ass. D6
Slaanesh inflicts instant death
40 pts well spent IMO.
Unfortunately lack traitors, so my old gaming buddy wouldn't have use for his hundreds of IG anymore. (cheapest troops is marines at 15pts a pop)
They've also gained a few units that don't count towards force organisation... such as demons and spawns.
The new plastic minis range is amzing as well. the commander/sorcerer was great to put together, and I managed a demon prince using possessed parts.