In truth I have been largely unimpressed with almost all of 8th edition's mechanics and concepts. However, for the sake of dialogue and in the interests of this thread, I will focus on just a few that especially stick out to me. As a quick disclaimer, I am largely influenced by (older) background lore and thematic elements over in-game actions, and this does affect a lot of my reasoning.
In terms of Tau specific mechanics, I think at the moment my biggest disappointment is how drones have been handled, in particular their new target selection rules. Since their inception one of the big aspects of Tau technology was that their AI and robotics were always a cut above the rest, second only to the Necrontyr in such areas. Thus, where Imperial AI controlled units like Tarantula sentry guns or the various kinds of Imperial robots had their tabletop actions dictated by strict algorithms, Tau drones were, if enough were networked together, able to operate completely autonomously with the freedom of a living unit, attacking whatever targets they were commanded to. This made sense to me.
Now, they have been saddled with the same 'can only shoot the closest target' rule that Imperial sentry guns had, which given their aforementioned AI sophistication doesn't make sense to me. Perhaps more worrisome than that however, is that I can't help but feel it's somewhat restrictive on their battlefield role. You see, going by background lore, with the exception of Shield Drones taking bullets for living troops has only ever been a secondary function of drones. Remember that in Tau doctrine, NO unit is truly expendable - they try to avoid drone losses as much as any other. Sure, if it comes down to the terrible choice of picking what doesn't return to the base in one piece after the battle, the Tau will prioritise living units over the drones, but they still don't see their main purpose as simply being there to die for others (aside, of course, from Shield Drones, which are specifically built to intercept and absorb incoming fire). Gun drone squadrons, for example, have always been used just as much as mobile strike elements, zooming along a flank to eliminate enemy manoeuvre elements or being dropped behind enemy lines on search and destroy missions or leading an advance into hostile territory (that third battlefield use even comes from the original Designer's Notes for the Tau). With these new rules for them, however, I can't help but feel like these major battlefield roles have been neglected in favour of pigeon-holing Drone Squadrons into the one-dimensional niche of bullet sponges.
Likewise, the new seeker missile rules also don't make a lot of sense to me. While I understand the idea of requiring a markerlight hit to use them at full effect (though I'm not sure why at least two are needed in all circumstances), firing them at the carrier's BS seems odd to me. According to background lore, unless a seeker missile (or one of its ilk) is being used in a dumb-launch, the carrying vehicle never has anything to do with their aiming - once the carrier gives consent to release (and not even that in older background), the seeker missile homes in on the markerlight beam using its own onboard automatic guidance system. The previous seeker missile rules that gave seeker missiles an independent BS of 5 (or had them hit on a flat 2+ in all circumstances, which was largely the same thing) was a good reflection of this. The current system of using the carrier vehicle's BS subject to modifiers, on the other hand... not so much. Again, seeker missiles and their ilk are supposed to be self-guiding towards markerlight returns, why would they be affected by the aiming abilities of the carrier vehicle's crew?
The current paradigm of plasma rifles now being worse in all respects to Imperial plasma guns instead of a lateral shift trading stopping power for reliability and operator safety also bothers me, but that's largely a consequence of making Gets Hot! optional on Imperial plasma weapons.
Pulse weapon stats are a similar bother for me. I understand why their AP characteristic was removed from a game design perspective, but it feels silly to me that a class of weapons that's always been touted as the last word in small arms firepower, described as "Punching through armour like a comet through a dust cloud" in the earliest Tau overviews and going through armour "with contemptuous ease" in the Tau section of the 8th edition rules indexes, has difficulty overcoming even the lightest and weakest infantry armour in the setting.
I also sorely miss JSJ. While the current 'move out of close combat without penalty' setup is a passable effort to emulate it, it ultimately falls flat for me. While the tactical opportunities it provided were considerable, the real brilliance of the Jetpack rule and JSJ ultimately was that it gave Tau players something to do in the Assault Phase (in Warhammer Fantasy the Anvil of Doom and to a lesser extent Runesmiths have a similar importance in giving Dwarf players something to do in the Magic Phase). In later editions Overwatch mitigated this value somewhat, but it only ever activated when charged which meant that it wasn't really an activity for Tau players when it came to their own Assault Phase. The JSJ paradigm, on the other hand, was a stroke of genius in this regard - not only did it make the Assault Phase useful for Tau players, it also allowed them to be proactive in the Assault Phase instead of only reacting to the opposing player's assaults, and in a fantastically subtle but important way; if the Movement Phase was one of the most tactically important, then turning the Assault Phase into a second Movement Phase would give a huge advantage to a Tau player... but only if they were able to make the fullest use of it. While the new system does consolidate all movement into one phase, it also leaves Tau players with very little use for the Assault Phase and, perhaps even more crucially, takes away a means for a Tau player to make some active key moves of their own in the Assault Phase, making the Tau more reactive instead of proactive on the table. Sure, you can deliberately charge a jetpack unit into an enemy and plan for them to get out of it next turn, but not only does that feel like an unnecessary risk to me, it also doesn't make a lot of sense to me from a background perspective - I don't think I've ever heard of Tau outside of the Farsight Enclaves voluntarily entering into close combat with an enemy before.
There are many things I dislike about the core rules (especially the morale system), but I feel like I've vented enough negativity for one day, so I will limit myself to one thing: templates. I deeply miss templates and the scatter die. The newer mechanics may elminate arguments, but it just feels so much more satisfying to drop a template on a big mass of enemy troops. I also like the visual spectacle of it more. You may scoff at such an irrational criticism, but it has had a noticeable effect on how much fun I've been having playing games.
Finally, as a Warhammer Fantasy die-hard I also find the AOS terminology creeping into 40k deeply irritating.