Just something that shows how much music you've been listening to for the past 2 weeks or something, some statistics.
you can intergrate spotify with last fm. which does just that thing, and very well. however i would like it if spotify made its own statistics database with something like total time listened to each genre (which can be seen with some add ons for last fm)
While not exactly a native Spotify feature, if you've got your account linked up to Facebook you can get listening stats a little bit like this.
And of course there is the Last.Fm integration too, as chives2 says.
I agree though that it would be nice to have stats in-client. I'm a little bit of an obsessive myself.
I would love listener stats. I use them all the time in Media Player. Helps me rediscover tracks I haven't listened to in a while.