Biggest thing is the improvement of Fire Warriors. Yes, they are better. They still aren't worth your points or filling your entire Troops slot with but they make a fairly decent rearguard scoring unit now. With the change to Rapid Fire weapons allowing them to be fired at full range whilst moving, Fire Warriors can out-duel most other basic infantry for a time at range and Rapid Fire earlier. Their S5 is also a bit more useful with the introduction of Hull Points - they can chip away at lower AVs in midfield though they aren't likely to drop anything in a single round of shooting (except maybe 2HP AV10 vehicles without cover in Rapid Fire range). Again, a nice boost overall but still not a fantastic unit.
Which brings us to Kroot. The reduction of general cover saves hurts Kroot, particularly forests. Cheap units with poor saves really benefitted from 5th edition cover, it was really easy to get a 4+ for the entire squad without needing them to be sitting in cover. 3+ if said cover was trees or if they went to ground and a 2+ cover if they went to ground in a forest. For 7 points a model. Reduce each of these by one in general and add in cover is determined on a per model basis and Kroot protecting a Tau army are a lot less effective. Chuck in the ability for an opponent to dictate where models are removed from with careful positioning and holes can be punched through bubble-wrap a lot easier (a Commander with LoS! rolls though can mitigate this). This is bad for Tau armies who really relied on assault armies being slowed down by such. Kroot also lost some harassment utility without being able to Outflank and assault. Not a huge loss as their most important task was to protect from assaults but it had some offensive use which has been lost. At the same time, this is a general boon for the Tau army as assaulting outflank units are much less threatening to the Tau army.
Speaking of cover...the impressive Tau shooting gets a lot better with its reduction. Not only are vehicles easier to tackle (and Tau had NO problems with this before) but mass infantry is less of a problem and markerlights are much more capable of stripping cover and giving BS boosts to units. Options like the plasma rifle with low AP values become a lot more useful and with options such as focus fire, how cover is determined (per model) and the ability for the shooting player to determine when saves are taken, such shooting is a lot more powerful.
Furthermore, the wide range of medium strength firepower Tau have to offer is generally better against vehicles than before thanks to hull points. The combination of S6/7 from Crisis Suits can reduce vehicles up to AV12 to scrap metal pretty quickly and Railguns are still the premier weapon for tin can openning at range (improved with average lower cover saves as well). The average S5 weapon throughout the army is also capable of pinging off hull points here and there as discussed above with Fire Warriors. However, with the loss of target locks, overkill from Tau firepower is more of an issue (i.e. Broadsides) and an attached character has less options throughout.
With one side of the coin we give to Tau shooting, with the other we take away with enemy assaults. With the weakened ability of infantry blocking and increased potential charge ranges, Tau can find themselves in assault potentially earlier. Doesn't matter generally who they are against, Tau suck in combat so this is generally pretty bad. Add in the general reduction in effectiveness of bubble-wraps and assault can be reached much more reliably for the opponent.
6th edition has also seen a general increase in objective based missions and focus on scoring power. This was never a strongpoint of Tau's as they preferred to break opponent's on their backfield shooting than move into midfield. Let's take a quick look at units and Tau options then.
Blacksun Filters - cheap and easy to access across the army. Great for ignoring night fight and allowing Tau's strength (shooting in case you didn't know) to shine.
Drones - extra wounds for the important units (suits). With the new closest to closest way wound allocation works, extra points spent on drones will increase unit survivability a lot.
Disruption Pods - every other skimmer got a free 5+ cover save so it sucks that Tau lost an advantage though everyone knew this was coming in some form...
Bonding Knives - even less useful than before.
Retro-thrusters - give any unit Hit & Run. Potentially some awesome combinations with Battle Brother Allies (TH/SS Terminators, Seer Council, etc.).
Ethereals & Special Characters - lol no. Ethereals still make your army panic when they die, Farsight can no longer have mass target locks in his squad though might be an okay ally and the other SC's never really did anything anyway.
Crisis Suits & Commanders - their firepower is much more powerful but they can no longer split fire with target locks. New assault move is more likely to leave you screwed than not but it can helpl you get away from units in a pinch. Excellent unit to ally into another army.
Stealth Suits - still overshadowed by Crisis Suits which bring the important S6/7 firepower to the tabletop.
Fire Warriors - better than they were but still not great. Not a dead weight in terms of a mandatory ally and can in fact be taken twice as cheapish backfield scoring.
Kroot- still decent firepower (improved with Rapid Fire changes, justlike FW) but less capable defenses.
Devilfish - good front and side armor is useful for pushing midfield but Fish of Fury isn't coming back, even with Flat Out moves - useful tactic at range though and can potentially counter poor assault moves for Crisis suits (though you'd need to pre-empt this). Otherwise still cheap and decent firepower.
Pathfinders - less useful with the reduction in cover - both for their markerlights and their own durability but still a nice option.
Vespids - hey my AP3 is better! Care face.
Piranha - same as before and with infantry blocking less useful, vehicle blocking will be more important.
Hammerhead - less vulnerable to suppression or having main weapon knocked off. AV13 plus hull points is quite durable at range as well but still suffers from a single shot when shooting at vehicles.
Broadsides - lost out with no target locks but still great antit-tank and great as allying unit.
Ultimately this leaves Tau still relying on a core of Crisis Suits & Broadsides but still quite vulnerable to mass MEQ armies (i.e. Blood Angels, though FNP is weaker to mass firepower but stronger to AP2), assault armies and winning based on scenarios. They are also lacking in the psychic department. Luckily, most of these holes can be fixed through Allies!
Space Marines & Eldar can be taken as Battle Brothers and Tau love some prescience action on Plasma + Missile Crisis Suits. The retro-thruster Shas'El can also give Hit & Run to any units from those armies which has several options. Unfortunately though, Eldar still suffer from poor scoring and assault defenses so still aren't a great choice. Space Marines are a much better option here though.
Beyond that, any army which can add to scoring and assault defenses is the best bet fora Tau ally and reallyrequired to see them do better. Imperial Guard can replace Kroot with Guard Platoons and bring extra firepower through the likes of a Russes, Manticores, Vendettas, etc. Orks can like-wise do the same but with a more aggressive approach using Ork Boyz. Deathwing adds scoring and scary Terminators. Necrons can provide flyers but nothing really super in the scoring or assault defenses department. Same with Grey Knights and Black Templars re: scoring - generally better choices with other MEQ books though Strike Squads fit nicely with the shooting of Tau and scoring Paladins can be amusing. Blood Angels bring Mephiston, Stormraven & a fast Vindicator and Troops similar to Vanilla Marines but aren't Battle Brothers. SpaceWolves can fill a more aggressive MEQ ally role with Grey Hunters as scoring Troops but they can be left out to dry a bit without other support units.
Ultimately, Allies are needed to fill the holes still seen in Tau lists. I imagine even when the Tau codex is re-done this will be the case but hopefully not so extremely and there will be more choices in the Tau codex proper. That being said, those good choices are so good they make good additions to other lists who already have a solid army on which to build.