The missing feature for me is the ability to share a column between boards. This would let me work with other people. Work is hierarchical. I make big decisions and then those big decisions get split up and consumed by other folks. I can't have everybody controlling boards at every level. So when I drop "clean room" to my "Stuff my son is handling" column, I don't care or know how he breaks that up and does it. All I care about is that once he's broken up that work and completed all of it that he passes the card to my "done" column. I create and track the task at a high level. He does the same at a lower level. My big task breaks up into a bunch of small tasks for him. The same kind of scenario plays out in lots of things: refurbishing a house, planning for a summer vacation, tending to landscaping around the house, and so on.
Everything can't be flat. There has to be some kind of hierarchical structure. Having one column appear in multiple boards would allow that to happen. Might be some other solutions. Don't know. You could get into some kind of parent-child thing, but that's butt-ugly. I think just letting one column exist on multiple boards would solve the problem without creating a complexity problem.
That's the one thing preventing me from active use. If they could fix that, and somehow figure out the hosting/privacy issues many companies have, it'd be a perfect app. Keep up the awesome work, guys!