Thanks for this thoughtful response. But:
5. Indeed, at least AJAX enables somewhat sane masking of this, but the only-one-request-per-response character of the protocol means a lot of things cannot be done efficiently. If HTTP had allowed arbitrary server-side HTTP responses for the duration of a persistent http connection, that would have greatly alleviated the inefficiencies that AJAX methods strive to mask.
Well... what's wrong with using HTTP 1.1 persistent connections? They do allow multiple arbitrary HTTP responses over a single connection, efficiently.
I'm coming here late, but after reading the comments I still don't see the problems with HTTP. There does seem to be a lot of misunderstanding of the protocol and its history, though.