Having been a beta-tester of DragonDrop from the start, one thing I'd like to add is how important the concept of user-interaction truly is and how Mark really innovated in this regard. Lots of Mark's friends, including myself, suggested using one of the screen corners as a hot-zone. But since most people usually have other actions associated with the corners and the extra distance required to drag the icon far from the center makes the movement tedious, Mark kept looking for a better way to handle it.

He spent weeks & months trying out different UI and actions, finally coming up with the 'shake-your-cursor' action. It seems trivial now that you see it but it was something none of his potential users could imagine. And since then, he's been refining the app constantly to get it to its current state, including the rotating background and optimal thumbnail for a variety of mimetypes. I'm so glad to see the app finally getting the attention it deserves.