Lacking? Pfft. You haven't seen lacking until you bought the Gundam Unicorn Blu-Ray. It has one episode of Gundam Unicorn-- one-- and nothing else. No extras, no nothing. What a waste.
As for what I like to see... Well, I enjoy previews, so that's my main thing. Honestly I don't watch too much else like cast interviews and such... Though I do like to see character biographies, outtakes, deleted scenes, and things like that.