Actually I'd say (and I think Joel, the inventor, said the same thing) that it's a meta tool which is much more versatile than a bug tracker. It's like Excel - people are starting to use it in ways nobody has ever imagined it. I admit that I never imagined it to be used as a household management tool. Of course, one can do Kanban on it, too. Oh that reminds me I have to check out their API, I'm quite sure it's going to morph into some kind of authoritative meta list store.