The font size.
Really. Getting old enough to *not quite* need bifocals makes the book really annoying in that tiny font. Especially because you aren't always playing int he best lit rooms...
Personally, I would consider that more of a "game-breaking flaw" than a "frustrating quirk"; but that's just me.Full healing after a long rest mostly.
1. The armor table. I mean it's incredibly ludicrous and arbitrary to begin with (why is ringmail now a heavy armor, for one example). But the cherry on top is that they got rid of banded armor (which historically existed), but kept splint armour (that didn't as a suit, and only then occasionally as pieces that covered limbs).
Every class is a caster.
Used to be a criticism of 4E I hear often enough but now it’s literally true - you have to go out of your way to NOT cast spells.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.