It's incredible how such a simple idea could become so big. I'm sat here trying to think up some grand scheme that's going to make me my millions. Its a great lesson that really all I need is a simple idea. The key is in the execution. Take that simple idea and develop (and market) it to perfection.
it's not a simple idea, it's a deep idea refined over decades and implemented simply. http://en.m.wikipedia.org/wiki/Kanban
It's really clean and straight-forward to use, but the simple components provide a lot of flexibility and power, while being easy to teach someone.
I use trello for just about everything, and I would have dropped it a long time ago if it took more than five minutes to show someone how to use the fundamental concepts. I can get them up and running in no time, and the users (even non technical users) tend to find all the interesting bits on their own as they go.