I will try any book, but it has to suck me in right away. If it has grammatical errors or writing goofs, it needs the pull of a black hole or I'm out.
I love to read and to write great characters. I want a ride, with all the jerks and gasps and thrills of a roller coaster and none of the motion sickness, though I'll take whatever the author makes me feel for the character gladly. Bonus points if I learn something.
Nonfiction books usually make up at least half my TBR pile, since that's where the world-knowledge I need lies. Current favorites include The Complete Idiot's Guide to Hinduism and Anthropology for Dummies.