Founders fall in love with ideas. The market never does. An idea is a description of something that could exist; a value proposition is a claim about someone else — that a specific person, in a specific situation, has a problem painful enough that your solution is the one they will choose and pay for. Those are not the same object, and the distance between them is where most early-stage time is wasted.
The confusion is seductive because an idea can be evaluated alone, in a room, by the person who had it. A value proposition can only be evaluated outside the building, against the indifference of people who did not ask for it. "This is a great idea" is a statement about the idea. "This solves my problem better than what I do now" is a statement about the customer — and only the second predicts anything.
This is why a great team can't save a weak proposition, and why technology can't substitute for a market that isn't there. Execution amplifies a real value proposition; it cannot manufacture one. Pointed at a problem no one urgently has, a brilliant team and an elegant stack just build a beautifully engineered answer to a question nobody asked — faster.
The hard discipline is to hold the idea as a hypothesis about a customer, not a possession of the founder. What problem, for whom, replacing what, at what cost of switching? A proposition that can answer those is testable, and therefore improvable. One that can only be defended by re-explaining the idea is not a value proposition yet. It is an idea wearing one's clothes — and the market undresses it on contact.