for example:
Subject: Sparrow already does virtually all of this
Body: The only real differences are the Subject thing (not sure how this saves you time, other than pressing TAB once), and displaying images from the address book.
Anyway, I would think that most of the time suck in email comes from replying to others' messages (which already have subjects), not creating messages from scratch when you're not even in the mail program. (If you do that a lot, maybe you should stop increasing everyone else's email burden...)