Preamble: I don't believe D&D in general, or 5E/5E24 in particular, to be a bad game in principle or a failure at what it sets out to do. If played with the right group of people, most RPGs are fun. I'm currently running 2014 5E for a group of newbies that specifically looked for a D&D game after they played Baldur's Gate 3. They're motivated, they're attentive, they're creative, so playing D&D with them is indeed fun and the system doesn't get in the way of that too much.
That being said, here's a list on the pet peeves and larger things that make me prefer to run (or, rarely) play other games instead:
The D20 is way too powerful. If the average result of a die roll (10.5 in D&Ds case) outweighs the attribute+proficiency bonus of characters for most of their adventuring shelf-life, the lines between competency and dumb luck blur too much. Related to this, I tend to prefer pyramid-shaped or bell-curve probability-inducing dice roll systems to solutions with a single die (e.g., 2D10, 3D6, or Dice Pools instead of a D20) as this further removes arbitrariness, and enhances the importance of choices on skill advances (the one thing that I miss from 3E D&D in 5E).
Too little coherence in the "fluff" parts of the game. It's a multiverse, there's 50+ origins, anything goes, you can be whatever you want to be. I understand this is part of the general appeal of D&D, however, for the sake of providing yet another option, any idea of balance gets thrown out of the window (Hello, playable species Aarakocra). Yes, I'm aware most of this is technically optional and could simply be banned by the GM. It still offloads the task of maintaining coherence in worldbuilding to the GM, who has to decide what to allow and what to ban in order to not turn the game world into a circus tent.
For a game that aspires to be more than a mere dungeon crawler, there are far too many spells that only have a stated distinct use in combat, and far too few that have a stated distinct use outside of it. In general, magic (and magic users) feel a tad too powerful.
If the optional rule of multiclassing isn't allowed, the progress path is far too narrow and rigid. Every meaningful choice in character development is done latest at level 3 out of 20 (at least theoretically, I'm aware most groups don't play that long). If the optional rule of multiclassing is allowed, you can kiss the ragged remains of power balance in your group goodbye.
Official publications for 5E have been very much hit-and-miss for me and appear to differ a lot in atmosphere, thematic direction, target audience, art style and general usability. Book binding, paper quality and to a certain extent also reasonable page count (Hello, Spelljammer) are lacking when compared to releases of other decently successful RPG publishers. WotC / Hasbro appears to be more focused on developing the brand "D&D" and less on writing a good RPG book.
There is a glaring lack of consequence to being wounded (if one understands "losing Hitpoints" as being wounded), or dying (at least at later levels).
I'm not a big fan of an arbitrary limit of Spells per Day. Why is that the case? Does the wizard simply "forget" how the fireball spell works after they have just cast it three times in a row?
There might be more that doesn't spring to mind right now.
Again, I'm aware all of this is mostly personal taste and not objectively bad, neither is this necessarily "ruining" the game for me, just makes it somewhat less appealing.
