* Charts (of course)
* Recommendations: I use last.fm for two kinds of recommendations: (1) where it uses listener history to figure out similar bands and albums, which in some ways is inferior to feature-based recommender systems like Pandora, and (2) where it uses neighbor listening history (e.g., neighbor radio) -- I can't count the number of times I've been introduced to awesome bands in genres I wasn't looking for just because a neighbor had it on their charts.
* Shoutboxes for each artist, album, and song: I can't count the number of times I've just instantly "loved" a song and needed someplace to shout something out with others who like the song.
Last.fm has a huge user base and should try to be a one-stop shop for music, like Netflix is for video. Here're some tips
* Revamp your business model: copy Grooveshark's business model if you have to, but get anyone who wants to listen to a song come to your site and not go to Youtube for a low-quality version.
* Increase avg. time on site: If you implement the suggestion above (where I can play any song I like), then you have a wealth of _real_ related song/album/artist to offer the user. You may be able to remove annoying audio ads and just rely on click ads. It also helps if you implement a site-wide list like youtube's playlist.
* Improve recommendations: Netflix had a decent algorithm, but conducting the Netflix prize got them a lot of publicity _and_ a much better algorithm. You could do the same with the ton of data that you have. Yahoo! is already ahead of you in this: see this year's KDD cup: http://www.kdd.org/kdd2011/kddcup.shtml