Ones which solve real pain points.
But it's the wrong question to ask, YC heavily focuses on team over idea, the idea is a validation of the team as much anything else (i.e is the team smart and capable of thinking deeply about a space).
But it's the wrong question to ask, YC heavily focuses on team over idea, the idea is a validation of the team as much anything else (i.e is the team smart and capable of thinking deeply about a space).