One of the things I love about Remember the Milk is the Firefox addon to gmail. For example I can see my day's tasks right beside my email.
However, the best thing is the ability to quickly add tasks. This fits right into my GTD lifestyle. Someone calls with a task, type it quickly. I think of something while triaging email, type it quickly. No having to switch screens with the inherent possibility of getting distracted. I go back later to the full RTM page and complete needed details for the quickly added tasks. Thanks RTM!