Turn-on: I have bought one game based on graphics alone. But it was more curiosity through how the art-style would work. The game was XIII, which was based on a comic, and the first game I saw with cel-shaded graphics. I was so intrigued to see how this would work in a FPS so I got it. Mildly satisfied with the game, it's not really a purchase I'd regret, however was the only one based on aesthetics. Other games I've bought it has been a significant factor, though never a deciding one. I still play quite old games, and try out quite a few independent titles that don't even catch me in a design or creative sense. Not shortage of fun there.
Turn-off: Generally the aesthetics never quite turn me off completely, though they might be an indication that it's a kind of a game that I simply don't play. Most JRPGs, cute kids games, sports games, etc. I have played at least one of each of those and enjoyed though. The one that is really a turn off and I avoid like a ten foot pole is the MMORPGs which have anime style exaggerated proportion characters and skimpy armor. Funny thing is, it's not the skimpy armor that puts me off, it's the likelihood that it is tied to very average, tedious, grindy shallow gameplay (I actually love bikini armor and busty characters in appropriate *fantasy* setting, hell I nearly bought Dragon's Crown based on it's crazy aesthetic style alone).