When I see a good looking cover, I'm going to look at it more readily than a bad cover. A good cover can be something showing a group of characters doing something (like the cover for Exalted or Jadeclaw or AFMBE), or it can be iconic and suggestive (like Vampire, Mage or D&D).
But I won't buy a game based on the cover alone. It's got to be something I'm interested in, which I'll determine by the back cover blurb, the game contents, the artwork, all that stuff. It usually does give me a hint about the production values, the amount of graphic design attention it got, that sort of thing. Lots of squishy feelings, for sure.
Few covers outright turn me off, though I know them when I see them. More than anything, the cover sort of acts as a psychic call. Does it stir my imagination? Do I gape at it, and just enjoy it's beauty before even thinking to look at the back? Is the rest of the book going to enspell me the same?
It's so personal and quirky, though. "Beauty in the Eye of the Beholder" and all that jazz. I love Changeling, but I love the design too - the stain glass motif. Even if I hadn't know what Changling was going to be before it was released, I would have gravitated to the cover simply because it had stained glass. Same for the Amber Diceless cover. I had never read the books, but seeing the unique tarot cards and the fluid painting itself just drew me in to see what it was all about.