-The ability to make time estimates for each task
-Nice email integration, with the ability to reply to notification emails to add comments to the tasks
-Number of task and hourly counts for the columns
From having a quick look around the service, I think I'll stick with Trello for my projects, but then time tracking isn't something I need. I can see that this would be quite useful for many freelancers though, who need more time tracking capabilities. And at any rate, it's nicer than Pivotal!
Email integration is a good idea, and something that Trello is really missing out.