Practically all of them is my guess. As of last April, when
Steve was starting to work on RSS for Blogger Pro, he did a test feed with the description in a CDATA section, and at that time it failed in NewzCrawler, Feedreader, and AmphetaDesk (i.e., every RSS reader I had at the time).
Aaron Swartz's feed is using CDATA in content:encoded, and although I've been too lazy to formally test it, I suspect that's what keeps killing Aggie for me. It's the Right Thing To Do, but someone needs to do some serious evangelizing before it will be a safe thing to do.