Interface
1) Resizable columns, sortable columns (i.e. i can't sort by "Sender" right now), and pluggable columns (If i want to replace say, Internal-Date with X-Sent-Date). It would help to have these fields available in the Search Builder, too.
2) Tasks. Sure, it would be great to have this integrated with a custom forms and workflow engine (as it seems that's your projected-path), but its just, well, missing right now. -- Along these same lines, I'm eventually hoping we're getting a Notes-like forms builder--thats the plan, right?
3) A way to script popups/mashups into the system via the Admin UI.