I always read what the book's about and the very first paragraph. If those two things don't draw me in, then I'm not buying it. I especially hate a novel that starts out with a long, descriptive sentence about a "place" - a sentence that finds 40 different words to describe a sunset over a lake. Ok, the sun is setting, we're at a lake - get on with the story!
I also look to see how many copies of the book are on the shelf. If there's more than 5, and the book or author is one I don't recognize, I don't give it a chance, well, unless the description and first paragraph are amazing.
Length is important, too. I don't want a skimpy book, but I don't want one that's over 800 pages either. 350-650, average length I prefer.
I also like books that have more than one part. Don't know why really...
OH, and I also have a few "cover" quirks. I hate it when the author's name is 4 times bigger than the title. And I hate "commercial" covers - covers that are plain looking, have regular text on them and one small image in the middle - Danielle Steel's novels come to mind (I don't read them). My mother reads them, and I saw one that only had a tiny heart that had a "crack" in it, on it. I mean, what is that? A huge name like Danielle Steel and the publisher hires cheap cover designers? I mean her covers look like they were thrown together in 5 minutes.