I used Feed Validator to check my appEngine RSS feed, and fixed all validating problem. It wasn't easy.
One problem was that dates need to be in RFC-822 Format. Another problem was the Email address which should include real name : "yonizaf@yonizaf.com (yoni zaf) ".
The last problem was guid. It's a tag that has to be unique for each item, but also be a valid URL unless stated otherwise. I ended up using the timestamp for that. oh well.

Checking around, there aren't many feeds that validate. Even FeedBurner made an error: while using ?format=xml they didn't include it in the Self reference, resulting in a validation error.