From the moment I first started using it, everything just worked the way it should, incredibly intuitive, none of the bloating or over complicated functionality that other project/task management platforms suffer from. For me the killer feature it the 'tree structure' approach to task management, this is how it should be done, take a big task/project and break it into smaller and smaller manageable tasks. Each subtask has the same feature set as the main task which is exactly how it should be done because why should any task have less importance. The interface is clean and functional, everything is where is should be, it just works. You have Organizations, then projects, then tasks, which is a great way to organize things, especially if you are freelance like me and do projects for a number of different organizations. I am a long time trello user, and for anything looking for an alternative that does more than just kanban, this is for you, it will allow you to really structure tasks and make sure they are followed up.
No Kanban is going to be a big negative for many people, but apparently that is coming soon. Interface might be too simple for some people, or perhaps lacking features such as chat, message board, rich text markup, etc etc, but that could be seen as a good thing by others. Pricing has not been released either, that is something to be concerned about , especially if you invest time in adopting this platform but then it turns out to be overpriced. That is probably my biggest concern at the moment as features can be added all the time, but pricing can be a deal breaker.