Joel spells it out in his announcement[1].
In the future we'll continue to add free features to Trello (there is a lot of exciting stuff in the hopper)—anything that is a common feature, useful to anyone, will be free. We’ll also continue to develop new Business Class features that help large organizations manage Trello, and we may come up with other things to sell to people who are getting a lot of value out of Trello. In the meantime, we sure appreciate the cookies!
The current version of Trello is pretty feature complete as far as I'm concerned. More stuff just creates reason to mess around with Trello instead of doing actual work.
They have effectively created two classes of users which they will support differently. Business is going to get features that the free plan will not. Deciding how they will split their time between the two users bases will be one of their biggest concerns going forward.