"What is the thing you like most about reading book blogs? Is it the reviews, author guest posts, articles, giveaways, or something else entirely?"
The thing I like most in book blogs are the books. Book blogs introduce me to the books I don't know. It is a very easy way. With their reviews or thoughts about it, I can save my money to good books only. Also, I like interacting with others. Blogger around the world, in book blogs, interaction is possible and free. Of course, the contests. I'm not that rich that I can buy 5 books every week. With the help of the contest, they are helping with my passion.
In short, I like the blog itself and all it contain and who runs it.