This looks really cool. I'm excited to try it out, individually and with others.
Even better that you can immediately sign up and give it a try. As opposed to Asana, which has similar intentions, but is doing a private beta with larger companies only. Yes, Trello is much easier to get excited about.
EDIT
Some feedback: so far I really like it. It's intuitive. And it has some nice features out of the box that are lacking in other products such as assignment and voting. Assignment allows central authorities to exist, as well as hand-offs between people. Voting is awesome. I immediately see two uses for this: democratizing, and to allow collaborators to vote on what they want to do. The latter being something I've always personally wanted in a collaboration tool. Combining that with a central authority can be very powerful by allowing people to voice their interests, yet keep the project moving and avoid conflict.
As for the initial reaction: some of the views are pretty intimidating. Even thought there's only three lists at the start, it is still a little much to begin with. A lot to take in. If you ever implement a "minimize list" feature, that could easily reduce the noise for a beginner while still allowing you to explore all the features when you're ready to.
The edit screen has a lot happening in it too, but I think that's less of a problem since it's only visible when actually editing cards and you'll soon be using everything in there.
Also, I'm sure it's already in the works, but keyboard navigation will be huge for upcoming power users. Right now Vimium provides that for me, but it'd be great to have it built in.
It looks very useful! I'm already having fun with it and I'll try it out on an upcoming collaboration. Is there going to be a UserVoice (or similar) site anywhere for additional feedback? I'll happily leave this and other feedback there.