"Further, the output format ought to be determined hubristically from the HTTP Accept header; that lets clients select output formats without resorting to ugly ?format=xml crap."

I have to disagree. First of all, do not assume anything based on mime types the browser (or whatever user agent) asked for. It's very hacky to expect consumers to mess with their user agent's Accepts header to get the response format they want. Specifying the desired format such as ?format=xml is simple, clear, specific, and reliable. I prefer default format be XML and other formats available by passing a format parameter.

The REST of article has some good suggestions. I made a pun there. Did you catch it? "REST of the article". Because it's about REST? That's comedy gold.