It's not clear what Phaser has that Trello doesn't, other than more specific names for things, and the Arc idea, which doesn't seem specific to software, other than the name.
- create and group many stories at once with indentation. you can optionally use Vim for this
- stories have statuses that keep track of state: Started, Finished, Delivered, Accepted, and Rejected.
Those two things and Arcs are what make it different. I plan to add more features in the future which will make it more specific to software, such as robust statistics and predictions around story completion.