My problems with D&D 5e:
1) combat is BORING. And yet in so many ways the core of the game. Every fight I've been in up til now starts with the opponents doing major damage to the party (helped by a GM who cannot stop rolling criticals for the life of him

), and then halfway through the balance shifts and we destroy the opposition. Also, despite all the conditions, every player either says something like 'I hit it with my sword/I cast Spirit Guardians/I nuke them with Fireball'. And monsters are all the same (even when using say Kobold Games' excellent monster books, the end result is just another bag of hitpoints coming at you with one or two distinctive attacks). I want combat that is exciting, and to me that is visual and dynamic (roll for a parry, let every character attempt common combat actions like a disarm or a parry, use criticals that are spectacular in effect etc.). It's the one point on which I really want something different from what D&D provides.
2) I don't like that certain actions/situations which occur all the time in the fiction (and in real life) are made impossible by the rules. Holding someone hostage with a knife to their throat, punching someone out with one good or lucky punch, grabbing someone's arms so they can't attack or cast spells anymore...
3) The removal of the surprise round now seems to mean that you can never get the drop on your opponents anymore. Whether one is very alert and careful, or whether you just suddenly attack out of the blue or cast an offensive spell, the DM always asks to roll for initiative and that usually ends up really bad for me. This ties in with the previous point: inability to do things that should be possible, because of the rules.
4) two actions, reaction (sometimes), bonus action (sometimes), a move action + an action = 2 move actions but not = 2 actions... I find this far too complicated still, and too chaotic. I used to feel that one action per round is more than enough, but now I'm charmed by the three actions and that's it-approach of Pathfinder 2. Keep it simple and efficient, which makes it newbie friendly and streamlined.
5) the eternal hitpoint situation (no DM has ever said to me when an opponent hit me: 'you are feeling a little less lucky and are a bit more tired now'). It's much better than before on low levels, but the HP bloat on high levels makes combat boring again (see point 1). I feel that high hit points at starting levels and a far less rapid increase later on is a far better way to treat the problem.
6) Stats: I really don't like the fact that stats can keep increasing. Almost every human warrior is stronger than an ogre. I feel that 18 should be the top stat for a human, and only magic can increase this. And non-humans can go up to 20 in one or two stats, but should be limited in others.
7) The option of a pure class, without subclasses which, let's face it, are basically prestige classes, should also be present and viable.
Those are the main points. But the combat is the biggest problem.