Many thanks to all who gave advice. I learned a lot about some campaign settings I was interested in, as well as about some I had not even considered. In fact, I want so many of those suggested that I have decided to implement my own personal "Campaign Setting a Month" tradition. The plan is to get one new book each month, looking back on this thread to help me decide.
March is officially Oriental Adventures Month, because it was high up on the list and easy to find this weekend. In the coming months I will see whether I am in the mood for The Hunt: Rise of Evil, Nyambe, Scarred Lands, or something else.
The Scarred Lands deals are tempting, so I may go ahead and spend a little more now.
Looking through Wheel of Time, which was originally at the top of my list, reminded me of one crucial difference: WoT is meant to be a d20 game, not just a setting. The money spent on that book goes largely to reprinted material, much of it from the core books. It still looks interesting, but I will probably wait until I find it at a discount, or even on ebay.