Well-written characters. It can have the most boring plot in the world, but if the characters (and interactions between characters) are well-writen, I'm down.

Writing style.

I am down for any book, as long as the author can play with the language. It can be long winded as Wallace, or poignant like Vonnegut, or poetic and soft like Haruf, or dry and almost scientific like Asimov, or logic bending like Pratchett, but all these authors can use language like an instrument. This property doesn’t make a book easy to read, but makes it most definitely worth my while.