Setting sells to me.
I've been known to buy games based on setting alone, without even knowing what the core mechanic is.
The main reason is that I don't really know how the mechanics will really work until I play or run the game a like dozen times. Even if I know the core mechanic, how it meshes with the setting and how all the sub-systems work are hard for me to visualize without playing the damn thing a few times. So, when in the market for a game, I don't spend too much time worrying about mechanics. That said, if I'm familiar with the core mechanic and I hate it, then that kills the sale.