I won't buy a game if it doesn't look good. Now, let me just clarify that I don't just mean graphically, although that is also sometimes a determining factor. I often do a bit of research on games I'm interested in before buying them, so I know what I'm getting into. Screenshots, videos, and positive reviews are generally what it takes for me to realize I'd like to purchase a particular title. So if the game doesn't look like I'll actually enjoy it, it's not going to be in my collection.
Speaking of which; why in the various incarnations of hell did I buy Wet? I've gotta trade that sorry excuse for a videogame in already...
Also, generally, I won't buy a game if it's practically designed for online multiplayer. If the game has 4-player local multiplayer, then it's a plus in my book, but when a game is obviously designed for online multiplayer, as well as offering an eight-hour long tutorial for the multiplayer in the guise of a 'single player story' mode, then something isn't right.
I also won't buy a game if it's on the Wii.