The many ways in which you can use it, very flexible. And that they're listening to their community of users.

I can...access from my phone, laptop and work computer and it syncs effortlessly; create different lists to keep work and personal stuff separate; use tags to easily view tasks related to the same project all in one place; set a date to start working on something (this was a feature that other apps lacked), set a priority; estimate the time it will take to complete each task; set recurring tasks; quickly add all the details of each task with keyboard shortcuts; view what's due today, tomorrow and in the upcoming week; create smart lists to see what I need to start today or in the next few days or see only tasks that take a certain amount of time to complete; search for tasks; and get reminder emails or texts for tasks. I also find it quite cool that RTM will tell me how much time my tasks will take for each list and each day (based on my individual estimates). By the way, I get all of the above on the free version!

Ease of use, intuitive interface, and simple aesthetics. Great synchronicity with mobile and desktop apps as well as browser version.