I think that tags with which everyone can contribute is tricky, tags usually seem to work better if you can have your own taxonomy of tags and they contribute to the site. (eg: delicious lets you tag stuff, and browse those, in addition to everyones tags.) With an implementation of that scale, I can think of more valuable features to implement (like combatting the decline in comment quality recently)

Really not sure what tags would achieve over Google and/or http://searchyc.com/