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.
You are expecting gmail to act like a desktop app, and maybe you could make the argument that it should, but that's a different issue.
IMO bigger problems are that email ought to be plain text by default and replies ought to go in the bottom (with three dots on the top if necessary).
Sorry if I made anyone mad. Now you know how I felt reading this post.