Big Things:
A lack of mechanical rigor.
Lame monsters. By lame I mean uninteresting. A battle with an orc chief and a bunch of orcs is only interesting because the orc chief is an interesting monster. The generic orc isn't... and they're basically all the same too. You could create an interesting scenario, but said scenario would be even more interesting with more interesting monsters.
Unbalanced monsters. I don't necessarily mean monsters that are overpowered or underpowered. Sometimes a monster can even be both! There's a problem with mechanical rigor. A monster might look interesting on paper, but in play you find a monster (say a ghoul) is far more powerful than other monsters of the same level. Playtesting obviously helps fix this, but there's such a lack of rigor that these problems aren't instantly detected.
I don't like human stats or ability score boosts versus feats.
I don't like a lack of roles. Once again, the fighter, wizard and maybe thief are all competing to see who can do more damage in a fight.
Certain elements of spellcasting. Rituals are nothing like 4e rituals, which I saw as "long term cast for long term gain". In fact, I'm not a fan of any non-4e-style buffing spells. On the plus side, Vancian magic has been fixed to an extent. Wizards get at-wills, and they no longer have a tremendous number of spells per day at high-level, helping empower low-level wizards and hit high-level wizards with a desperately-needed nerf stick.
Save DC/saving throws/bounded accuracy. It's not working, and I don't think WotC talks about it anymore. To me, bounded accuracy means your poor saves will always be weak, and your AC will not increase (along with attack bonuses, which actually scale a bit) unless you're a Dex-class. So we'll have high-level rogues, rangers and monks that are untouchable by the monsters, as WotC cannot really predict what a high-level PC's AC score is likely to be. I especially do not like the monk's AC system. It's exactly like in 3e, and you have a class that starts off with terrible AC and ends up untouchable. This is bad exactly the way a weakling level 1 mage becoming an incredibly powerful level 9 mage is bad.
This works in reverse too. Monster ACs don't really scale, and there's nothing they can do about poor saves. A hill giant is never going to logically have a high Dex score, leaving it incredible vulnerable to Dex-attack spells... which will scale, because casters get to scale their save DCs in two manners (a level bonus, plus always boosting their key stat). Sometimes that's warranted (an extremely clumsy zombie or a blob of protoplasm shouldn't be dodging anything) but not here. (Is it worse if hill giants get an unrealistically high Dexterity score? IMO, yes.)
Magic item guidance: WotC is probably stuck in an impossible position here. Required amounts of magic items results in a "must collect and save loot" scenarios like in 3e (and 4e without inherent bonuses). But if you don't have guidelines, you get a Monty Haul 2e scenario instead. No matter which way WotC goes, they'll tick someone off. Personally I'd like to see number-boosting items die in a fire, and then far less guidance would be needed, but that's a sacred cow, and 5e is bringing back a lot of that.
Healing: There's a "Healing Word" spell, but it's really weak. Instead, you need to rely on crafting, or Cure Light Wounds, which is again ye olde boring "spend a full turn to spend a standard action to heal plus move up to the healee" which makes clerics incredibly boring to play. While there's non-magical healing, and faster than in most versions of D&D, it's still really slow. You still need Saint Bandaid in your party.
There's a bunch of minor things I didn't like: rangers still casting (WotC can never get the ranger class right, as it's not really even a class), weird wildshape rules, kender in the core rules (that's a big thing for me but a minor thing overall), and multiattacks. One thing I was happy about in 4e is you don't get multiple attacks due to levels, so your (much more complex game) can avoid at least one element of gumming up the works.
