Today I was pointed to the following: "Last.fm has thousands of tags, unfortunately they are all pretty bad." (A statement made in this video of a very interesting talk about autotagging and applications that can be built using tags, around minute 51.)
I think this needs some clarification: Last.fm has a lot more than just a few thousand tags. The 1,000,000th unique tag applied by a Last.fm user was earthbeat about half a year ago.
Related links: fun stats on Last.fm tags, Last.fm's multi-tag search.