What's the App Store refund policy? The Apple FAQ has this same question, but doesn't really have an answer. To quote:

What is the Mac App Store refund policy?
If you experience a technical issue with an app, first contact the developer of the app for assistance. Their support contact information can be found on the app's page in the Mac App Store. If that does not resolve the issue, contact Mac App Store Customer Support.

So the policy is that I contact someone for help... but can I actually get a refund? Suppose I buy an app that claims to play avi files, but it only works with three out of four files I throw at it. Would that kind of problem typically result in a refund?
Well, in my point of view, if the app works 75% of the time playing AVI files, I would try to contact the developer directly, there might be something required (like a plug-in) or a bug that needs fixing. Like it says in the FAQ, if contacting the developer produces no result, then I guess you could ask for a refund.

I have heard that they gave refunds to people who were unhappy before, but it was a last-resort solution if I remember correctly. Also, I presume the App store staff will fight for you more if you paid 100$ for your app, and a little less if you only paid a buck... :-)