One thing about tags which I think is worth a try is to make them up/down-votable. It's one thing to say: "this is interesting" and it's another to say "this is on topic". Current sites based on tags doesn't allow for much user input in the latter area.

I dream of a day when I can customize youtube to not display anything video game related (when searching for highlights from sport events and stuff). Everything tagged as "sport events" and in fact being video game footage would be downvoted to death and users posting this wrongly tagged stuff flagged as spammers so there would be incenvite for them to tag things properly.