Warning: Spoilers in the following link
http://dvd.ign.com/articles/533/533531p1.html
Thats a link for details on all the new scenes in the extended edition. It also talks about what happened at comicon and some other stuff. I didn't read through it all, but I did kind of skim and one thing caught my eye.
Spoiler:
A fight between Gandalf and The Witch King. I was a little disappointed at the lack of wizardry in ROTK by Gandalf. Hopefully this fight will make up for it.