Some thoughts:
I don't buy or use PDFs. I want a printed book (or a printed book with a PDF version of the same book would be nice too).
I'm not currently interested in a new campaign setting. I play in the Realms and I find those products to be very good values and used quite a bit.
I'm not currently interested in a new system of magic. I actually don't want to learn a new system if I can avoid it. If one of my players does and doesn't mind coaching me through it when it comes time to put it into practice, that's fine.
I would buy a product that addresses a gap in D&D. So when I get to some seafaring adventures I probably will pick up one of the seafaring rulebooks.
I do like a few new spells now and then, and sometimes new critters -- spells from the Books of Eldritch Might and monsters from Green Ronin's fiend books have seen use in my games.
However, WotC has also put out some books that I personally find less than useful: Deities & Demigods is one; Manual of the Planes was 'ok' but not all it was cracked up to be in my opinion; Faiths and Pantheons was too much like Deities & Demigods and not enough like the 2E Faiths & Avatars; Epic Level Handbook will see zero use.
Things that can turn me off to a publisher or its products: poor or non-existant relationship with the community; general unprofessional behavior (I know, it's not fair, but I watch how they behave in all kinds of threads, not just in the Publisher's Forum); poor production values (as I say, I like the actual printed book as well as the info inside it); lack of attention to detail in products or on their website; any hint that they know better than I do what is "good."
There are dozens of d20 publishers and hundreds of products. I can afford to be very picky!
