Basically, the design philosophy writ large.
'Ask your DM' is not game design.
I have no idea what they think 'natural language' is, but... this ain't it, chief. It just adds more ambiguity seemingly to encourage people to ask their DM.
The cruelty of knowing there were better designs in D&D Next and we'll never get them.
Levels 1 and 2 are pretty empty and you aren't really the character you're trying to be in most classes (subclasses). This is meant to create 0-levels, but they're too strong to be 0-levels and too weak to be fun for people who don't want 0-levels.
Bounded accuracy makes leveling not feel like advancement and acts like it's a feature that you get to fight the same boring monsters forever. All to make the math simple, which to me is not a worthy design goal.
We can't have just setting books and just options books because of the business model.
"Math is hard, let's get rid of static bonuses. Let's also add randomized bonuses like +1d4."
'Optional' features means we can half-ass them: magic items, feats, etc. Oh, you want a good magic weapon that's not a sword or ax? Go fish. Want decent magic economy or craft? Don't make us laugh. Want feats to have an interesting character design? We have one for you. One.
Tons of ASIs, but max is 20, which you can start with your primary at anyway. Stat boosting items set your stat, so you wasted your ASIs, chump.
Daily Attrition design has reached its apotheosis and they're planning on doubling down on it.
Heavy dependence on subclasses and a stubborn refusal to admit some chassis can't handle everything and just make some new classes.
Design space is self-limited. Advantage/disadvantage for EVERYTHING.
Monsters. Where to start? CR is a cruel joke on new DMs, spellcasters require you to get another book, Too few cool abilities.
Combat healing isn't actually worth it.
6 saves to make things 'simpler' than 3... somehow.