Got the DVD of HP&theCOS yesterday (like $15.99 at Best Buy, and my sister got it cheaper than that at Target, plus a free 2-liter of Coke, so shop around).
The big news is: They let you go right to the deleted scenes and other features without having to jump through any hoops. The most significant of the deleted scenes is an extensive one with Mr. Malfoy and Draco in Knockturn Alley. There's also a funny one with Lockhart giving his new class the Pop Quiz on himself, and praising Hermione for knowing what his life's ambition is.
Haven't watched the JKR interview, yet, but there are some cute snippets with the adult actors (loved mild-mannered David Bradley showing us how he puts in his false teeth and becomes nasty Mr. Filch), and a section with the kids answering questions about making the movie. In the production sketches section, there are some drawings that are clearly from the "Nick's Deathday Party" sequence that didn't make it into the movie. I was disappointed that the "tour" of Diagon Alley didn't give you more options to investigate things: I really wanted to go into Eelops' Owl Emporium and have them show and tell us about the various kinds of owls, but no such luck. Anyway, overall, though, I think they've made this set more "user friendly" than the first one.