"Does it work, and who cares?"
Christensen's insight is that customers don't buy a product, they buy (or "hire") a solution to a problem. In developing a new product or service we should always focus on what real world problem we're aiming to solve. It's what my training mentors at IBM used to call "solution selling" and it ties in nicely with my personal, pragmatic attitude: I don't care as much about technology and systems and processes as about solving a real-world problem.
So, what's the problem we're trying to solve? Does the product or service actually solve the problem? Does it do so more cost effectively than some other way?
Is the problem we're solving sufficiently painful that customers will pay to solve it in the way we suggest? Will sales and distribution people care enough to get the solution in front of customers?
This is another important nuance on this week's insight that marketing is about solving customers' pain, not pushing a product that we've developed in isolation in a lab!