At Yahoo! Research Berkeley, we investigated the hypercontexts around sharing and linking videos through social constructs. Last year we claimed pragmatics are more important than semantics. It was a pretty bold research move (but was typical of myself and my tall colleague).
The core of whats happening here (and hopefully Google knows their purchase is indicative of this) is we dont always consume media for content. We watch videos to be conversational. We watch videos to add or listen to commentary. For example, violence during a soccer match can be conversationally funny—we saw the hilarity ensue. YouTube’s annotations were a start but I didn’t see much come of it. Maybe Omnisio will help. The tricky part is making something useful available to people, be it in sync or async. Oh, and you should checkout Andy Baio’s use of Viddler; its my favorite video tagging usage to date.