Colin Devroe knows what he likes and doesn’t like about Mail.app, although he says that “I’m not as much of a power-user of Mail as I probably could be.”
He has produced 
a list of gripes and a modest list of wishes for Leopard Mail.
I see what he means when he asks for more flexible searching:
The search box should allow for multiple filters such as you find in the current Finder. Searching for a subject, then being able to click + to drill down until you find what you are looking for. I have about 12,500 pieces of email, and finding the 1 that I am looking for can sometimes prove difficult with a single search filter.
It would be great at the click of a Finder-like plus sign to search for emails from a particular sender with a particular word in the subject line:
That’s much easier than the Boolean search “hack” for Mail.
The complaint about smart mailboxes is also right on target. And well-observed; I’d not noticed it before.
When you create a smart mailbox and select “message is in mailbox” you get a list of your existing smart mailboxes. When you try to create a “message is not in mailbox” criterion, you don’t. Why not? He wants it fixed.
His modest feature requests — an iLife media browser and the automatic compression of multiple attachments — are not what I would choose, but it’s great to see someone thinking outside the box and coming up with features that add functionality not just eye-candy.
A fine post.
