aphexII -- how non-functional is file transfer, in your opinion?
yakko -- tabbed windows is an interesting idea. I like it.
ggavinmoss -- Cross-referencing an address book might be difficult, but it's still an interesting idea. Perhaps you coud send your address book to a central server and it would return buddy names for any registered email addresses? Or were you thinking even bigger than that??
The reason I ask is b/c, as some of you may remember, I was telling AT members about the new BitWise chat that my friend and I have written. It's come a long way, and, in fact, the offline message emailing is something we have implemented. The reason for this thread is to determine what killer features are missing from existing services, because without anything truly new, there is little reason for anyone to switch. So I'm polling for the gaps.
In any event, feel free to check it out at
http://www.bitwisechat.com. Messaging, file transfer, storing away messages, invisibility, chat preferences and many other things are already implemented. Plus it's small, about 300k, and has a really small memory footprint.

And it runs as a stand-alone program, I use it in the labs here at school with no install!