My favorite thing about used bookstores is that the books are cheap: I can try out new authors (based on nothing more than the cover and the blurb on the back) and not feel I'm wasting money. This goes double since I'll be trading in my previous set of novels, so those new authors are even cheaper, and I won't have any compulsion to keep the books around.

My second-favorite thing is browsing a (relatively) small selection; Amazon is so amazingly huge that I'm not sure where to begin, but browsing a dozen shelves of <insert interest here> is fast and easy.