Big ideas are scary. That's the gist of what I've got from Paul Graham's keynote speech at PyCon US 2012 and "Frighteningly Ambitious Startup Ideas"
Don't be afraid of big ideas, instead attack it from different angles in smaller bites. Start small and eventually you'll get there.
Oh and check out the rest of the videos from PyCon US 2012
Offtopic: Spotted on HN, some guy actually took the time to analyze Paul's 'uhms' per minute in his PyCon speech. Well played good sir.