I allow anything published by WotC. I suppose this leads back to my work as a game designer. I like to get practice playing and running the elements of all the books, though I'd hardly consider myself an expert in everything. I'd still like a lot more practice with new systems like ToB, ToM, and MoI. There are occasional elements that I don't allow, but for the most part, I'm an easy DM in terms of books. It's up to the players what they want to make. My games are equally difficult and challenging for all kinds of characters, though I admit that I don't have much room for housewife commoners with Skill Focus for all of their feats.