"What is the thing you like most about reading book blogs? Is it the reviews, author guest posts, articles, giveaways, or something else entirely?"
I do like every aspect listed in the question about book blogs. However, the thing I like best is the sense of community that I have found to exist with book bloggers. I also like how blogging has made the world a more easily accessible place. I love that I have made new contacts - and even a couple of friends - with people that I otherwise may never have met. In fact, one of my closer, real life friends and I bonded over books and blogging. I am grateful, on a regular basis, for the way that book blogging has enriched my life.