One option I'm missing is as simple as reversing playlists (often they are in counter-chronological order when they shouldn't be) or that I can't order the subscribed channels on the left panel manually (I have so many subscriptions, that I have to get to the subscriptions manager, scroll all the way down and go to the second page to find some of my favorite channels) or infinite scroll which makes the viewport jump around when using the back button jump around and generally feels slow and clunky (although YT's implementation is not the worst out there at least.)
The player itself is quite nice here on Chromium (not HW-accelerated but that's a longstanding Chromium/Intel bug), I agree. Otherwise the site is relatively fast and reliable; I like especially the video streaming smoothness and automatic quality detection, which is lightyears ahead of anything I have seen elsewhere.
They ought to have a clear indicator of thumbs up/down ratio on each video thumbnail (a couple pixel bar at the bottom colored green/red would easily do the trick). That would save plenty of time that would otherwise be spent watching some troll video or annoying commentary/reaction edit of some other person's work.