I used to work with Bill at Seven, and this article captures his personality pretty well.
His biggest strength is he's an unbelievable salesman. He can talk pixie dust and make you want to sign on to his vision.
I believe the ability to sell your vision is the most important skill for an entrepreneur (which is too bad for me, cause I'm a coder and I suck at this skill).
One reason I think the advice in this post is valuable is not just to test the startup idea, but also to test the founder's appetite for traffic acquisition.
Many technical founders like building products, but those products will fail without the majority of energy directed toward getting traffic. So if you can't get a couple hundred email addresses of interested customers, you can save yourself a lot of time by not going further.