Interestingly they offer blogs and tagging (they seem to have learned the lessons). The personal radio now can be used with a separate client (which is not yet available on OS X, so I can’t test it). Anyway Last.fm is a very useful way to get introduced to new music I probably like: I just have to browse the playlists of my neighbours.
I remember a similar feature back in the Napster days in 1999/2000: you could browse the music libraries of remote users that seem to share your music taste and learn about artists you never heard before. Last.fm and iTunes Music Store would make a good team. The “users that bought this also bought that”-pattern is too simple to be a real value.
Update: Last.fm just released an OS X Player.