One feature of BeOS that makes the platform appealing to graphic artists is its alpha channel, which builds transparency controls right into the operating system. Grab any icon and drag it around over the top of other open windows or other system elements — the icon will appear translucent. For an extra-groovy demonstration of the alpha channel in action, open any image in ShowImage, drag out a small-ish selection (no larger than a couple of inches). Drag your selection around over the image, the Desktop, or other open windows to see a ghostly, translucent version of the same image.
The degree of user feedback offered by this feature is amazing. On the surface, it seems purely cosmetic, but it really does have a way of making the system appear to come alive. Pure elegance.