Very overlooked yet powerful usability feature: Drag and drop file saving
It's one of the things that has bugged me about Windows since I started using it, the Open and Save dialog boxes. They're just unneeded. How do I know this? Both the ROX Desktop shell and Risc OS do without them.
Instead, when the user saves a file, they are shown a dialog box which allows them to type a file name. They then drag the file to an open folder window. An image of this is here: http://rox.sourceforge.net/screens/saving.png
This saves confusion and time. For one, if you are working on a project, it is very likely you have a folder window open showing the contents of it anyway. By dragging the file icon into this window, it is simply more human, you have a clearer memory of where it went and the shell doesn't have to try and guess where you want to save the file based on the last save.
When going to save the file a second time, you simply hit the OK button as the program remembered the previous path.
Haiku seems to be going beyond BeOS, a true fusion of *nix and human usability from the ground up, I think this feature could help it achieve this goal.