I'm familiar with the Bingo Card guy (Patrick McKenzie). I read his blog. What he's accomplished is definitely impressive.

I think the difference is that he's providing something teachers already use, need, and look for, as opposed to a "new" kind of software solution. (Online gradebooks aren't that new, but they still are not in use by the majority of teachers.)

Maybe I didn't hit on this hard enough in the blog post—teachers just are not a ripe audience for software in general.