I think there's a much better one: how easy it is to take your data and go home. With Trello, the answer is "exceedingly easy." On any board, just go to "Options" -> "Share, Print, Export..." and dump the entire board to JSON. From there, it should be easy to move your data onto Remember the Milk, Pinterest, org-mode, or whatever's appropriate for you. You can even automate grabbing that JSON on a daily basis via the API, if you're so inclined.
So while I understand your argument, your actual risk here is very, very low.
As another Creeker pointed out elsewhere in this thread, the Trello team has some stuff coming out in the future that you may find appealing, if you would feel better contractually paying for stuff. But in terms of risk, I think that it can be very low, today, whether you pay or not.
I hadn't even noticed the export features and now that I've taken I look a lot of my concerns seem much less important. Thanks!
Still trying to figure out how to effectively use the "due date" feature though. When a card's checklist is complete it still shows as past due. Not a dealbreaker at all though, we'll figure out how to make it work.
Disclaimer: I wish he was charging--I have a similar project I've been working on over nights and weekends. It is very hard to compete with free and difficult to position/market a product you have created to appeal to everyone. That being said--I love working on my project so it'll be around for a long time... Link: http://www.thetaboard.com
Every company can fail tomorrow. Free or paid.
Too bad they don't provide that.
I wonder how many versions of FogBugz they will still release as hosted version before they only offer it hosted.