What's usually the biggest mental hurdle for hackers to transition into entrepreneurs?
pg: Coming to terms with the effort required.
Still, it's important to remember that some people work two full-time jobs and barely make ends meet, while others work roughly the same hours and make millions. Working smart is just as important as working hard. I suppose that's a goal of YC's -- to help us work smart while we work hard.
Intelligence is necessary so you know what to do. Persistence because the universe is messed up and not always consistent, and doing the right things might sometimes not work for no explainable reason (but also because it's impossible to account for all variables and make perfect choices all the time, so we've got to be able to absorb that).
Think about it this way: you can be smart and know how to solve a very defined problem. But even if the world is deterministic, we're not close to being able to control it through pure logic and reasoning alone. That is we don't have the computer power or mind power to identify all variables and make sure we always do the right thing. THIS is why we need to persist. Because luck and unpredictability force us to try the right methods over and over again. Even if they are right, they may not work all the time because of luck.