First glance: A topic that interests me. Great artwork doesn't influence me that much. Really terrible artwork will if it's a work that depends a lot on art, like a monster book or a superhero book.
What makes me buy: Good treatment of that topic. Something I can use, especially re-use. Very good maps. Well-done NPC's (background, but not too much; a good illustration of that person helps). All in all, I'll tend to buy rules over 'fluff'; I can create my own fluff but don't want to go to the trouble of creating rules. Rules that handle things in simple, elegant ways I had not thought of will get my attention over rules that are just lists. A new magic system will get my attention more than another spell list. A new twist on feats (like, for example, ceremonial feats or regional feats) will get my attention more than a simple list of feats.