Have been spending a little time with my syndication feeds. I did have validation errors with a couple, and from my search of the forum, others have done so also.
First, within the Atom feed, I couldn't get the excerpt to work so I literally deleted all of the code having to do with excerpt and full content and just left:
<summary type="text/html" mode="escaped"><?php the_excerpt_rss(get_settings('rss_excerpt_length'), 2) ?></summary>
Second, the date format for the RSS 1.0 feed didn't validate until I added -05:00 at the end.
Had to do same with comments, too.
Third, the author email didn't validate for the RSS 2.0 syndication feed, and since I don't care about this, I deleted author.
The RSS 0.92 feed seemed fine.
After these mods, they all validate, and I'm only getting excerpts not full feeds (my preference).