Dive into Mark has a very comprehensive blog entry on the myth of RSS compatibility worth checking out.
I have often stated (1, 2, 3) that there are 7 different and incompatible versions of RSS. This was based on an embarassingly simple formula: I counted the version numbers in use (0.90, 0.91, 0.92, 0.93, 0.94, 1.0, and 2.0) and came up with the number “7”. But recently some people have taken to claiming that there are not 7 versions (despite obvious evidence to the contrary), and even if there are, that they are somehow “compatible” with each other so it doesn’t really matter. So I dug a little further to precisely document the incompatible changes in each version of RSS.