I just started reading Hatching Twitter after hearing about it in the best of episode of the Triangulation podcast. The interview talked about spotlight hogs, the rewriting of history, and how people who significantly responsible for things worth billions of dollars ended up unknown and without anything to show for their contributions.
The main thing I wanted to talk about is a lesson repeated in this book that I saw over and over in Founders at Work. That is this: Many very successful startups only become successful after completely ditching their "great idea" and going in a completely different direction. For example, PayPal started to make person-to-person payments via Palm Pilots, and Yahoo started as an online database.
What can we take away from this? Is it that the idea isn't important -- what brings success is getting a bunch of energetic, smart people working together to see what happens? I think that's true.
So if you want to start building something but don't quite know what to build, don't let that stop you. Just build it. Chances are you'll end up throwing it away and building something completely different. But you can't take a shortcut and build the different thing first, because you need the hard-won experience from pouring your very being into something that doesn't work in order to light that magic spark.