While I completely agree that generating lots of ideas is what makes great design happen, I also think that great designers, like Watson, do this mass-idea-generation in their heads, and constantly. Sketching, and making multiple prototypes, is actually a very very late stage in this process.
Speaking for myself and not for “great designers”, I’d say that for every ten sketches I make, or every three higher-fidelity mockups or prototypes, there are probably a hundred different possibilities flying around in my head, being evaluated, rejected, re-combined, and refined long before pen hits paper or before words exit my mouth.
Conversely, I suspect that people who are bad at design settle immediately on what seems to work well enough, and then proceed to plan for and build that.
The key difference is the instinct to reject — the idea that, no matter what seemingly brilliant idea you come up with, there *must* be something better.
Sometimes this happens in the same person’s head, but sometimes it needs to happen in a team, too.