As the thread title states: what do you look for in a book?
This question while simple can be hard to answer. I think it's a lot more difficult if the reader, for example, has varying taste. I'd like to think of this as a good thing, but reading the thoughts of writers, I imagine it must be quite frustrating. If the likes of Dan Brown can sell ridiculous numbers of novels, while author's like Scott Bakker flounder, you can begin to form a picture.
Personally, when I'm reading a book, I want it to have a good tale. Not a yarn, per se. I've gotten past those days. Not entirely, I still love the likes of 'Magician', the definitive yarn for me, but I require something more. It's hard to put a pin on what that is, but I guess that's what makes the difference between a great writer and a mediocre one. Being able to indentify this and make it work for readers.
The characters should be suitably grown up and well-rounded. I'm not hung up on a character needing to be interesting. I can accept that characters have different places in a story, some more suitable than others. That's why it can rile when you read a character acting in character, doing things you expect of him. It's galling when they suddenly seem to change and get driven by the narrative to perform seemingly impossible things. It can also be uplifting to read a 'weak' character in a story stand up for themselves and show something different. It's a fine balance. It works for some, not so for others.
Then it needs to have an odd human conundrum, to present a challenge to them mentally and/or physically. I love a good story in which the character overcomes adversity.
Anyway, this is rather difficult to answer. I'll get back to it further later. Need to brood.