- Search. Good, fast search over 7 GB of email.
- Speed.
- Undo send.
- Multiple aliases on the same email account.
It's definitely possible to beat Gmail in the first two, these days, but importing all that email into another service takes a while, so there had better be a good case that I'll get equal or superior functionality. A demo would be nice.
Until then, I'll stick with the old compose, and probably find or write a user script to make the new one work like the old one once it goes away.
(Incidentally, Mail.app comes quite close to meeting the first two, but its search is a bit wonky, and there's no undo send.)