I personally hate rounded corners and shadows added everywhere. Makes most things look crappy and smudged.
Lemmy is guilty of this too. I have a giant 4K monitor, why is all the content squashed into a teeny tiny sub-1000 pixel column in the middle of a sea of white? There is no reason I should ever have to scroll on a 4K screen to see a standard Lemmy homepage. Old Reddit got this right (same with mlmym) but default Lemmy UI needs the option to stretch. I have a userstyle installed that does this but it’s not perfect.
I think that everything is developed for phones, and then scaled to the desktop. I used to be you had to fight with a desktop site or app on a phone, now the opposite is true. (I honestly have no clue what I’m talking about, but from my experience it seems like that is the case.)