Update: An utterly basic missing feature that I can't believe Apple won't fix in Tiger is support for message priorities. I wouldn't mind better message flagging, either (multi-colored flags like Outlook 2003).
- A brushed metal UI (just joking, I like it just as it is...)
- A nice Outlook-like fading notification like Mail.appetizer.
- iSync capabilities with PDAs (c'mon, Palm users want it).
- Decent Exchange integration, not the hopeless (and, ultimately, useless) IMAP shim we got with Panther.
- A lot tighter integration with iCal and Address Book - think "meeting request".
- Type-ahead find (available as a plugin).
- More display options like Outlook's:
- "Auto-preview" (the short summary inserted within the message list).
- Message grouping by an arbitrary header.
- Staggered grouping by today/yesterday/week/month, etc.
- A vertical preview pane like Thunderbird's. Come on, I did it myself. How hard can it be?
- Better search (and pre-indexing).
- Virtual folders like Evolution's (and Outlook's, of course, but credit where credit is due).
And these are just off the top of my head. The niceties (and a lot of serious UI/interaction replumbing) will probably deserve more profound meditation by better-ranked pundits.