#1 - Drag and drop support for adding attachments - #1 end-user complaint - The ability to drag and drop when adding attachments would be a definite game changer.
^^^^^^This is by far my number #1
#2 - The ability to pass recipients and subjects in webmail url (eg compose.aspx)
#3 - I think #2 may help pave the way for a browser extension to handle mailto: urls and open in mailenable webmail