Paul's been digging up Copland links for me. Man, I feel all old skool. There's a fairly technical MacTech article from June 1995 which has some interesting nuggets (including the 'Assistance Engine', a kind of beefed up AppleScript to automate common tasks). But the real treat is this Byte article which discusses the features of Apple's modern OS before it was cancelled. This is the only article I've seen with a screengrab (which is what I'm really after): the file open dialogue box. Cool.
Most of Copland's features were rolled into later OSs: the new UI, the advanced search facility. Others have only come with OS X: Memory protection, good multiple users. Others were technological (like hardware independence), and we're not likely to see that. But the best features are ones that were brave refinements of an already mature GUI, enhancements that could only be done with a decade of user interface work already under the bridge: folders that could be 'Views', that is, dynamic searches, so a folder could contain all files ending in ".xls" which were "modified this week", regardless of where they were in the file system. There was to be an incredibly customisable interface, not just skins, but how it all worked, so more experienced users would have more functionality available. Wow.
I installed Mac OS X the day it came out, and it's now become my primary operating system. But there's a lot of work that needs to go in to tweaking the UI. Apple need to experiment, test, sit down with people using the interface and see how they can make it more intuitive. These barely perceptable features are what has always separated Mac OS from Windows, and making them takes time and effort. Microsoft have been tweaking since Windows 95 and they're doing a very good job. The only advantage Apple have now is a lot of lessons already learned; they just have to put them into practice.