Vision
What are you creating? What do you actually know about it? Who is your user, and what do they need to be successful? Then the harder one: what are you most in love with about your product — because that's where your blind spot lives. Steve shipped the Macintosh with no hard drive because he was fixated on beating IBM to market. Jay couldn't talk him out of it. It hurt the product for years. You never see your own blinders. The people around you do.