Quick task creation. There's no fuss in creating new tasks in Asana, no complicated processes, not buttons to click. Multiple task entry is as easy and hitting enter and typing the title of the new task.
Collaboration. You can collaborate with as many people as you want--in or out of your company or team. When a team member leaves a note on a task you're following, you'll receive an email and can reply directly to the email to comment.
Serious AJAX--I rarely have to reload my screen because Asana has taken AJAX to a new level. Even tasks that have been commented on via email will automatically update on my screen before my eyes.
I have yet to find a better task/project management app with as much flexibility, ease of use, and aesthetic qualities as Asana. I've tried 5+ other products in depth and nothing has compared to Asana.