I hate that enemies have so low armor class. In earlier editions, you had to be tactical, use flanking manoeuvres and charge attacks, prepare the right support spells, maybe even pick the Weapon Specialization feat for your favourite weapon. In 5e, no need; just stand wherever, roll an attack, you'll probably hit. In addition to removing much of the tactics from the game, this makes it basically impossible for enemy spellcasters to use duration spells. Good luck succeeding on 4 concentration checks per turn.
I hate that enemies' proficiency bonus is based on their challenge rating. No high-attack low-damage monsters here. Don't worry; the tank in your party will never need healing, any level-appropriate monster needs to roll ridiculously high on the dice to hit them! Everyone else just stay in the back and lob your bloody cantrips, and the battle will be over in 3 turns.
I hate that attack cantrips do as much damage as a weapon attack (or more). Why even have weapons at all, when your cantrips do more damage than a longsword, with better range than a crossbow.
I hate that cantrips scale with character level. No need to learn anything new for the rest of the game, your trusty Eldritch Blast will be your most powerful attack throughout. Especially when you get access to Greater Invisibility and don't need to rely on your bloody familiar for advantage on attack rolls.
I hate that familiars can do help actions in combat. Advantage every turn! And since they're no longer a class feature but a spell, they're also available to fighters and rogues, no multi-classing necessary. And unlike in earlier editions there are no real consequences of losing your familiar. All you lose is 10 gp worth of incense to get them back, a pittance at higher levels.
I hate that a long rest fully restores hit points. No need to ever stay in one place for longer than 8 hours, no need to conserve spell slots to do end-of-the-day healing, heck; no need for a healer at all really! And it gets worse when they reach 3rd level and get access to Leomund's Tiny Hut, and don't even need to find a safe spot to camp.
I hate that wild shape is basically useless in combat, due to challenge rating restrictions and the lousy selection of beasts in the Monster Manual.
I hate that the only logical combat use of Polymorph is turning into a dinosaur. Prepare for the inevitable discussion around the table: Can my character turn into a tyrannosaurus rex, even if they've never seen one? No? But, uuuuuh, they saw a picture of one in a book at the library!
I hate that you can use Counterspell to counterspell someone else's attempt at counterspelling your own spell.
I hate that any character can use any skill. No need for a rogue, just hand those Thieves' Tools to the character with the highest Dexterity, they'll get that door open.
The worst thing is that this game went through lots and lots of play-testing before it was released. The developers must have known about all of these issues and chosen not to change them, meaning that none of these are bugs; they're all features! This is how the developers intended the game to be!
Did I forget any of your peeves about the game? Add them in the comments. Alternatively, comment with what you love about 5e, let's add some positivity to this rant.