How do you decide the feature set for your MVP?

When it comes to building the MVP; the key is to define the feature-set we'll build for our MVP and stick to it. Often, we believe that we should build 'that one feature' before we launch and get into a trap. What's your way to decide what features to build for your MVP. Would love some examples of MVPs launched with limited features; but solved a problem that users paid money for. PS: Aim for this discussion is to find out real MVPs developed and the thought process that went behind before they were shown to public.