spam filters to *flag* spam is good, blocking spam is bad. Reason being, some people, like myself, would much rather get 100 spam emails than miss 1 good email, and you never know what someone considers a "good" email.
other than that, a quality search tool for finding messages would be good... umm, like gmail...
lots of storage... like gmail
a fast and responsive interface... like gmail
large file transfer limits... maybe a little more than gmail
pop3 access, like gmail.
Come to think of it, I guess just mimic gmail...
I actually have what your talking about (the outlook like setup), using Horde. If you could mimic the features of Horde (planner, tasks, calendar, email) add a file store (to be able to store files that I use all over the place)...
Oh, and make it extensible and brandable... I.e., let me offer co-branded email to my site visitors with my domain, utilizing your interface... split the ad space 50/50 including email taglines (like the ones that come on yahoo emails)... Kinda like the old offerings of everyone.net... let dozens of websites offer your services...
Anyway, those are my thoughts...
Brainstorming Ideas 
