Complex overlays. And making certain animations feel natural.
By that I mean that having an animated overlay like fire looks a lot better than some static vector png image.
But what really defines the créme de la créme for me, is using animations and overlays in a way that mimics more realistic movement. We already have x-amount of animations in the “catalogue”, but anyone who’s seen the animations more than a couple of times can recognise by sight what animation is which, and when you’re striving for realism and immersion, thinking “oh, that’s idle_hold_drink” or whatever can mentally take you out of it. But overlays that mimic hugs from behind, or a character drawing knees up to the chest, and such - actions we don’t yet have animations for - really make me take notice, especially if they’re effective and expressive.