I'd really like to see a Preferences option to make "Compose" open in a new window (similar to how most desktop clients work).
Often I want to compose a message, but perhaps it's something detailed that will take a while, and I'd like to be able to watch my inbox for other mail while I'm writing it.
Seems like it would be pretty simple - with two kids, a full time job, and grad school I just don't have the time to implement a patch to do it :(