I thought the Feed Validator’s ‘foo’ media type is not specific enough warning page was particularly useful/interesting.
RSS feeds should be served as application/rss+xml. Atom feeds should use application/atom+xml. If this is not possible, make sure that one of the following set of markers is present in the first 512 bytes of your feed
This post first appeared on From the Belly of the Beasts, a weblog from some of the people who build O’Reilly websites.