I think this is great, but I'm still unclear about why market researching your idea would have caused you to not pursue it further. If you had asked users when you first started, or even now, "would you use an awesome flashcard/memorization service that is FREE" I would expect the results that came back would encourage the idea of going forward with the business, because I know flashcards are a popular tool for studying and lots of people study.

Are you saying that the number of existing quiz/flashcard sites would create a research result that would discourage attempting another one? It seems like you knew while you were building Quizlet that you had something different in mind, so the only unknown was whether people would use it en masse. As it turns out, you were right, but, and this is a sincere question, was this vision or luck?

Isn't it more accurate to say that the art lies in knowing what questions to ask the oracle, and how to ask those questions, rather than ignoring the value of prophecy?