It was not well exposed, and unavailable completely on the iPad version of their app, but it was quite useful between my wife and I for sharing videos asynchronously.
It doesn't help that the Google+ profile they forced on me _also_ has a YouTube account, and I've uploaded some videos to the wrong one, so I get to do the dance twice.
It would be really useful to be able to go into a user's profile and filter out the videos you've already watched, or filter them at all.
It would also be nice to have a proper category list.
This is obvious functionality, that they must've left out on purpose. But why?
Clicking on "Youtube Studio (beta)" will show me some videos with checkboxes, and let me do some editing, I think, which is probably the thing I'm usually looking for.