It would be great if there was a way to show if you've already watched a video without having to open it. Other sites (PH for example) do this and it's particularly useful when using a headset because navigating is naturally a bit more involved than desktop/mouse/etc. Basically the easier it is to view a feed and quickly know "I've seen it" or "haven't seen it" - the better.
When I've noticed the need the most:
- After adding several (seriously a lot) videos to a Watch Later playlist, it can be hard to find where you left off in the playlist
- The inverse where you're trying to find something you already watched but didn't add to favorites/playlist.