Just like many file upload and photo sharing services (Shutterfly, Kodak Gallery), I should be able to drag multiple files into the body of the message (or into a special attachments dropbox) and then, using Ajax, the upload process should start automatically and asynchronously as I continue to compose the email.

I wish this would be added soon by the Gmail team as part of their Labs program. IMHO, this feature is more useful and important than (useless) features like Superstars, Old Snakey, Random Signiture and others.

But until this is going to happen, you might want to try dragdropupload, a useful Firefox Add-On which partially solves this problem.