http://hubski.com is in many ways a mashup between HN and Twitter. I created it from the HN source code. Building feeds built from following people and tags keeps noise down. People are pretty careful about what they share (retweets). I know Rob has an inactive account, so he is vaguely aware of it. He agreed to let me bend his ear about it (I lived nearby him), but left for the WaPo almost immediately after.
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.
Our approach is that the author can choose up to two tags, and the community (excluding newbies) can add a third. The most suggested community tag is the current tag.
