Validation
How do you know the market wants it? The biggest reason businesses fail is building something nobody wants or needs.
So we validate, constantly. Not "great idea, mate", that's worth nothing. Real validation is signals of intent: people doing something that costs them, not just saying something to be nice.
It's the hardest part, where most people quit, and where good ideas get killed by the wrong test. Which is exactly why you don't want to do it alone.