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.