As some of the folks that have been hanging around here for slightly longer might remember, I have an affinity for the Common Desktop Environment. While CDE sure wasn't pretty, it was a very consistent environment to work in, and it never surprised you; everything in CDE is where you expect it to be, and every action is followed by an expected reaction.
Anyway, the feature that I want to see brought back to life is minimising windows to the desktop as icons. In CDE, when you minimise a window, it becomes an icon on the desktop, which you can manipulate like an icon. There is no taskbar or dock entry for a window. What this means is that a window is an object, and you can manipulate it from just one place: the window itself, or the icon if it's minimised. Instead of windows being abstract containers for information, they become tangible objects in and of themselves. Especially with today's large screens, iconification (as it's officially called) makes a lot of sense, and we could finally do away with taskbars or docks nonsense.
Another interesting feature can be found in RISC OS. Instead of having a menubar in each window, or a menubar atop the screen, RISC OS put its menubar underneath the middle-click menu. Middle-click inside a window, and the vertical menubar would pop up. While this is a discoverability disaster, I still like it personally because it would allow for a cleaner desktop - I don't use menubars anyway (keyboard shortcuts!).
Moving on, let's talk Mac OS 9, for me the ultimate pinnacle of Apple user interface design. The Mac OS 9 'platinum' interface has a very pleasing feel to it, with lots of feedback upon interaction, as well as a highly consistent look. That kind of consistency is hard to come by these days. It's sad that Mac OS X can't even stand in Mac OS 9's shadow when it comes to clean, unobtrusive, and consistent design. It's a bit late, I know, but please Apple, bring back platinum! I'll just go back to my rocking chair and knitting needles now.
I want to give an honourable mention to Visi On, one of those early graphical user interfaces that should've gotten more airtime than they did. It had a very interesting way of ensuring portability: all Visi On applications were written for a non machine-specific virtual machine (Visi Machine), with only a very small core being machine-specific (Visi Core), using Visi C, a restricted subset of C designed for portability.
Visi On's files were dated December 1983, meaning it was designed at around the same time as Apple's Macintosh user interface. Even though it is mouse-driven and graphical, it has no icons and is mostly text-oriented. You can still play around with Visi On today, thanks to the floppy images preserved by ToastyTech and a copy of MESS.
What kind of features would you like to see brought back from the dead? What old or obscure GUIs do you find intriguing?