Ask YC: What heuristics do you use to judge ideas?

When you come up with a vision or an idea for a startup, how do you judge its merits? What questions do you ask?

Here are some of the filters I apply:

How does it make the world better? What does it make easier or more productive?

Who pays? Why do they pay?

Edsger W. Dijkstra said, "Only do what only you can do." I ask, "Why me?" followed by "Why now?"

If this startup executes perfectly, what is the maximum it could earn?

The Innovator's Dilemma (by Clayton Christensen) outlines the characteristics of a "disruptive innovation." Is this product disruptive?

Who are the competitors? If none, why not? Else how is this different? How easily could companies skilled in execution crush it? (Poor Kiko...)

How long will it take to complete? How will it be funded until then?

Are there any better, related ideas?

What's missing?