On a ever so slightly more serious note then my previous reply (sorry couldn't resist) I'd really like to see more products that are from, or at least based on, adventures and rules from the author's campaign or at least have been played by the writer/publisher.
I find these "types" of products much more fun to play and the writing less "stiff." (I wish I could come up with a few better terms to quantify my opinion here.) I can think of several obvious examples -- Tomb of Abysthor, Rappan Athuk, Monte's Eldritch Might material and The Lost Oupost (a good all around useful product that doesn't receive enough attention, BTW). I could come up with a lot of reasons (guesses) why this is so, but all in all, I can only say I've had a lot more fun with products that the authors state come from their own games and would like to see more of the same.
Also, I really have no idea how often writers/publishers play their own material and wouldnt' mind hearing more on that subject.
Walter