The historian Will Durant noted that the ancient Romans were fond of saying that "the start is half the deed". I have been thinking a lot recently about how prepared you need to be to before you launch your idea, product, company, etc. I have to admit I tend to lean heavily toward the "just start it" crowd. There is a saying floating around the entrepreneurial community something to the effect that "if you are not a little embarrassed by your product when it launches then you have waited too long to launch".
Not that preparation isn't important. But too much of anything can sometimes be dangerous to your health. Often we are presented with an opportunity that can't wait for the "right time". And no matter how prepared you think you are, you don't know if something is going to work until you try it. But moving quickly doesn't mean moving recklessly. You always need to understand and be able to manage the down side. Here is a good piece on all of this including how Richard Branson actually "launched" his airline.