No doubt some folks have expected me to post in this thread. I have largely stayed out of it, mostly because while I absolutely have my criticisms of 5e, I don't really know if going into them here is worthwhile.
In very simple terms, my problems with 5e are:
Martial/caster balance is back to favoring casters significantly over non-casters.
There is no simple caster class.
Healing is anemic, especially in combat.
Resting is poorly-designed at best and outright awful at worst.
The designers openly mocked stuff I loved about 4e (Warlords, dragonborn, tactical combat, etc.)
"Modularity" was vaporware, and the Tactical Combat Module doubly so.
The encounter balance is all off, both in terms of potency and in terms of frequency.
Skill use rules are poorly-handled and encourage 3e-style "every skill is narrow and specific" thinking. Huge step backward.
They killed Skill Challenges and didn't even talk about it.
The early game is the only place that makes onboarding relatively easy...and is stupidly deadly, punishing new players for small errors.
The refusal to just call actions Bonus Actions and Standard Actions.
Fighters, Wizards, and Sorcerers are all bland as hell and the first two do not actually support the thematic elements of their concepts.
Paladin and Ranger should never have been casters.
The Primal power source was awesome, shoehorning Druids (and Rangers) back into divine magic is dumb.
Feats were supposed to be awesome, big huge chunks of power. 3/4 of them are mediocre. Only the very best actually live up to the hype.
The DMG is, simply put, not very good. I dunno if I'd call it outright bad, but it's taken after the 1e DMG in all the wrong ways.
There are probably more things I could mention, but I won't. Suffice it to say, I have a laundry list of complaints about the design and implementation of 5e.