My first impression is that Trello isn't a (good) product (yet) but it will certainly leverage on Joel's marketing machine.
Observations: The organization information should not be public by default. I haven't found a way to delete an organization (is it there?).
The interface is unusable on iPad/mobile.
The card pop-up window is hard to use when there's some actual information attached to it. The bird's eye view is confusing and offers little information.
Activity log grows fast with information I would not need: voting events, add/remove members, etc. Make two activity logs, one with useful information and one with tracking (investigation) information.
In Opera and Chrome the red connection establishing notice appears all the time.
The in/out/public permissions are easy to use, but users may actually need more granularity.