Make a client with features better than LimeWire's anti-freeloader and xml-based meta information system, and I will love your software. :-) Other than that, here are a few things that are important:
-Make sure you can refresh a search or have the search results be updated automatically every once in a while (sometimes called passive searching)
-Make sure your software is able to easily accept new protocol changes and will not degrade the network with old Gnutella protocol.
-Make it simple, not simpler.

I know this may be a lot to ask, but you wanted suggestions.
;-)
Let us know how it goes, and Good Luck!