Hahaha oh your poor soul you don't know what you risk conjuring up with a question like that.

I have to write this all out in a blog post so I can just link it one day.

The core mechanic of 1d20+stuff produces flat probability. Every outcome on the die is equally likely. That's ridiculous. Go throw some darts at a dart board. Do you get an equal distribution around the board? Just as many hit the floor as the bullseye? No. So the underlying math is kind of trash.

The entire game is predicated on its rest cadence. You're expected to have like 5-8 medium encounters and then take a long rest. This generates a ton of problems for pacing and balance. Chief among them, most people don't want to play that way. Polls show people typically do like one fight per rest. Welp. Now all your long rest classes are over performing and your short rest classes suck.

Don't even start with "not every encounter has to be a fight". Don't even fucking start. Most people can't consistently come up with interesting non combat encounters in DND that tax resources the same way fights do. There are no real social conflict rules, for example, as mentioned below.

But even if you do somehow manage to do the suggested amount of encounters per rest, that severely limits the pacing of the story. There are so many hacks and variants to try to fix this. Gritty realism, sanctuary resting, heroic mode. They're all bandaids on a poor foundation.

The magic system is trash. It's just fucking bad. It had no real internal consistency. Every spell is bespoke. What's the difference between a third level spell and a fourth level? Fuck if I know. Can you make your own spells? Not really. Can you be creative with spells? Ehh kind of but they tend to be very specific about what they do, with few inputs.

Also like the way magic works is boring. There's no real flavor. You say you cast the spell and check off the box, and it happens. Maybe you need a material component. That's about it. It's shallow as heck. It's also weird that rangers paladins wizards clerics arcane-tricksters all basically have magic that works the same way. You could do so much more.

The social system I would say it was trash if it existed. You meet a pack of bandits in the pass. You want to fight them. The rules have a lot to say here. Hit points, armor, saves, actions and reactions, equipment, etc. Ok wait, you want to scare them off with your words instead. Well get fucked, the book has some vague guidelines that quickly turn into "the dm decides".

There are very few decisions to make about your character. Species and class. Maybe a feat or two depending on how long you play, but those compete with ASIs, and most games don't even get to 8th level. Subclasses sometimes have a few things to pick, but sometimes you literally get zero choices.

The skill system is extremely basic and you can't really specialize unless you're a class with expertise, and even then your options are kind of limited.

Magic items also have no real internal consistency. Why is the flying broom a like uncommon item despite being extremely powerful? Who knows.

Low level combat tends to also be very "I move and attack once". Some DMs might give you bonuses for taking advantage of the environment, but that's not well defined. It could be. It's not. Also making a single attack that has a like 40% chance of doing absolutely nothing sucks.

The main strengths of DND are brand recognition, and it's shallow enough that you can't really fuck up a character. Every human fighter is basically the same mechanically, which means your idiot 10 year old brother can play. But that also means you don't really have much depth to explore.

Pretty much every other part of the game is bad, under baked, or not suited for general purpose RPG stuff.

The economy is non-existent, actual balancing is literally impossible, you can't make a character yours (reflavoring your eldritch blast doesn't count), so many rules just don't exist or are on some random designers Twitter account instead of the damn books. If you want to argue it's a simple system; it isn't, it's stupidly convoluted for how little it actually offers.

Edit: look at Pathfinder (chosen because it's the closest comparison); it actually gives DMs a rough guide to how much money a player should be expected to have at any level, a decent idea of what players should fight in an encounter (2e even tightened that math up even more), a myriad of ways to customize your character on a real mechanical level, and all the rules are easily found on the same online resource. 5e doesn't do any of that.

it's an aggressively mediocre system that's had years of a huge community polishing it to a mirror shine.

You can praise it for the community content, or go off-book like you can with any other system, but that's applicable to any system with the same community size.

Whatever you look for in it it's lacking in comparison to another system. Tactical combat? PF2e. Rules light? Worlds without number.

It's a decent middle ground of a system only because of community hard work. But that's only for the GM side. Players still need to deal with the poor character creation, unless they get a lot of support from their GM.