OK, so that covers getting more data into Last.fm. Right off the bat, all that listening data is going to allow Last.fm to give you better song recommendations and help tailor your personal radio stations. But there's tons more you can do with this stuff.
If you head over to build.last.fm you can find hundreds of ways to do something cool with your Last.fm profile data. There are album cover screensavers, standalone desktop radio players, plug-ins for blogs and social networks, and options for showing off your listening trends using all kinds of crazy charts.