Oh boy buckle up because I have a laundry list
So IN GENERAL,
I think the system has a lot of ludonarrative dissonance. It attempts to invoke high fantasy tropes while limiting your power level so much that it feels like low fantasy. Plus, 90% of the rules deal with combat where probably only half of play time is combat at most.
I HATE advantage/disadvantage. I think it's a fine modifier but slapping it on literally everything in lieu of any other modifiers was a clear knee jerk overreaction to the stacking modifiers of 3.X and there has to be a happy middle ground.
If the class list is going to be so limited, the subclasses need to either be extremely flexible in flavor and function, or there need to be a metric fuckton of extremely flavorful options, and simply neither of these is true.
The spell selection is so limp dick, if you want to make a themed character without homebrew, good luck.
The weapon selection is horrible, there's usually a best in slot weapon for any build.
Martials get literally nothing to do with their turns, they just attack, bonus action if they're lucky, and pass.
Warlocks feel so awful to play because short rests are so inconsistent at 1 whole ass hour.
HP at level 1 is so fucking bad holy shit.
I've probably forgotten something because this godforsaken system is so awful that I could not possibly list all my issues with it in one post.