At work, a short comment in an email can very quickly build up into a large and complicated discussion. Lots of branching, occasionally hitting reply instead of reply-all, really a nasty mess. Part of the problem could be that I'm used to gmail and at work I have to use outlook, but I digress.
I think it would be really cool if once an email conversation kicks off, if I could send the entire thing to a resubj email address, and have it automatically build up the conversation up to that point. Then, the conversation could continue in resubj. If you implement this, or it is already in place and I missed it, let me know and I will start using your service immediately.
edit: also, if you set up some kind of api so that I could embed conversations on other websites, that would be kinda cool.