The ability to edit subject titles, and mail body contents would be nice. I have alot of people that instead of just replying to my mail message, they'll start a complete new one. Or sometimes, the message will get corrupted (usually happens with hotmail subscribers), and it would be nice to edit it. Not the end of the world, but annoying nonetheless. Entourage has this feature, and it's one of the few things I miss when using Apple's Mail.
Oh yeah....bring back the progress bar. I know you can open up the activity monitor, but that seems a little clunky. With that being said, I love Mail's simplicity, and for ease of use features, it trounces anything else I've tried.