To me, the kind of concept that's best fitted for bootstrapping is one that you can build in a few days and make money with right away like a project I did recently called DomainPolish.
I guess I should have been more careful about choosing the figures I cited, but I really didn't mean $50 million vs a billion as hard numbers. I was trying to say that as a bootstrapper you can target a smaller market or a smaller problem. That's something that you can't do as a venture funded business.
That means that all of the people who get funded early on who might ordinarily target your small problem are off building businesses targeted at HUGE markets. This means less competition for the bootstrapper and a greater chance to capitalize on the market opportunity.
I definitely realize how hard it is to do this stuff and I didn't want to make it seem like it's super easy to build a $50 million business. I'd love to actually some time man you seem like you have a good amount of experience with this stuff. I couldn't find your email but mine is dshipper@gmail.com. Would love to hear from you.