it's not a simple idea, it's a deep idea refined over decades and implemented simply. http://en.m.wikipedia.org/wiki/Kanban
Trello is about putting lists of cards on boards. That's it.
You can use it to do all kinds of things, but it's a medium rather than a framework. When I've tried other to-do list apps, or project management tools, etc. they all end up becoming too restrictive. They try to teach you to do the right thing in the right way, not realizing that it is different for different people.
Joel himself once talked about Excel, and how for most people, Excel isn't about spreadsheets, or formulas, or calculations - it's about laying things out in a grid. That's it. People just want to be able to write things in columns and rows, and colour them. And that simple behaviour is incredibly powerful.
100% this, for me.
That's how I'm able to get people to use it. They ask a lot of questions about what means what and assume it's more complicated than it is. As soon as they realize that the top level metaphor is seriously just a board with cards, they're like "Oh! that's really cool. I can use this" and they're off.
Some people use it for completely non-tech things, a couple I know use it to track their sales pipeline.
It's pretty amazing that there really wasn't something quite as simple made before it, it seems so obvious in retrospect. The implementation is spot on.
Now I want to write a Trello knockoff using homoiconic clojure.
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.
So it's a Lisp!
It can be used to implement more deep ideas such as Kanban, but these aren't forced on you.
I remember before Trello came out I needed an app that provided a list of lists (who doesn't). I designed it all out but never implemented it. I saw how much competition there was in the space. There are hundreds of tasks list and to-do apps out there.
So when Trello was announced I secretly scoffed at them thinking they were doomed from the start for choosing such an obvious and already implemented idea.
Still I signed up and started using it. It has been fascinating to see the app evolve and become a huge success. I only feel a little bit miffed that it was something I almost did myself since I know I would not have implemented it anywhere near as well.