For instance:
- add xmlns:atom="http://www.w3.org/2005/Atom" on <rss>
- inside <channel> ....

  <generator>
  <pubDate>
  <lastBuildDate>
  <itunes:block>
  <itunes:category>

I'm pretty sure they're not actually required by iTunes/iTunesU, but probably nice to have for atom compliance (and.. uhm.. what else?)