Something like that for YouTube would be nice also, I miss so many videos from channels I'm subscribed to because they don't show everything in your New feed. I would gladly pay $12/year for a service that does that and maybe a few other obviously useful features (tagging, attaching private comments, subscribing to friends public archives, etc).
Yes...tagging for grouping, notes for annotating, etc. There's also massive opportunity for crowd sourcing of topic based discovery.
The YouTube platform is amazingly horrible (from an end user perspective) considering how valuable its content is, if you could implement something that can't be blocked somehow I think it could be a financially rewarding little business, there are lots of heavy users of YouTube.
