I had almost the exact same thing happened. I bought Super Meat Boy for Mac. Checked the specs and I was above them. The game worked once or twice but then crashed every time I entered the level screen. I contact Steam who told me to contact the developer. The developer said that it was a problem on the latest version os OS X and wouldn't be fixed soon. They said I can get a refund but it has to come from Steam.

I actually showed Steam my email from them and still had to fight for it. In the end all they would give me was credit for the Steam store, they wouldn't refund me.

I thought this might have been a one-off (maybe I just got a bad customer service person) but apparently not. If I were a game developer I'd be pretty worried that the blame fell back on me for bad customer service like this.