Here's another total usability fail: Tab sends the e-mail you're currently writing instead of indenting text. Everywhere else tab indents text, and when you write an e-mail in gmail you'd expect it to act consistently with how it's used everywhere else. Start writing in gmail, hit tab to indent some text. Congratulations you've now sent half an e-mail to you boss.... I can't fathom who thought this was a good idea.
Nitpicker's corner: it doesn't actually send the email, it focuses the send button.
That said, after you hit tab, you'll probably type a word and hit space, which will then click the button and send off the message.