I browse not quite, but nearly exclusively within the Sci-Fi/Fantasy sections. I will meander into Fiction or History/War or the Classics on occasion though.

If a title catches my attention, I will investigate, or even an authors name. I don't even have to know it, but if their name stops me I feel obligated to check their book out. I also give an initial judgment to books covers.
If any of these things stop me, I will read the books description and maybe the first couple pages.

If I'm intrigued enough past that point then I probably going to get the book, if not right at that moment, then eventually. I actually just picked up a trilogy this weekend that I have been eyeing for the past couple months.

But one of the most important factors that I judge books on is their smell. I mostly shop in used book stores because I feel like the books have so much character themselves in there. There are certain authors or series that I only buy new from B&N, but the get a book that has already been loved and to give it a whiff, there is something magical about that.

I have found a book that I really wanted, but put it back until a different copy came to the shelf because it just didn't smell right.