A better email client - automatic classification of incoming mail, understanding mailing list headers, reminding me to add attachments if I've said "see attached", understanding bounce messages and tracking them against that address in my contacts list ("You're trying to email <foo>. I see that didn't work last time. Please check the address is correct."). etc.
If you make a better webmail-client, make switching easy! Allow me to give your site a try with my current archive of (gmail) messages and contacts, and keep gmail up-to-date, so I can switch back if I decide (and I'm a pessimist) that your service isn't good enough.
