User search, so you can search for a trustworthy user. With that goes a HotList.
Searches by Ping, and number of files shared.
If you are like Kazaa, maybe a feature to 'hop' nodes
Customizable interface? maybe (skins)
Usable browser, with an Address bar
Chat - Possibly integrate the "Messengers" into the program.
Just think napster, for basics
Sounds GREAT what you are about to do. hope it comes out nicley