If I'm looking for a comfortable read, I'll look at the genre and author first. If I'm looking for something new, I'll look at the blurb. The blurb should tell me what the story is about, and give me an idea if it's plot driven or character driven. (I prefer the former.) I lean towards stories that consider the boundary of science and society, and are typically being told in the present or near future. The tone of the blurb should show optimism for the future, and demonstrate that the author is trying to stay grounded in reality even if some elements of the story are fantastic.