Excellent post (a "ManifOSto") and very lively discussion at Signal vs. Noise about things an operating system upgrade should do (or not do). The discussion covers a lot of areas including future direction of desktop metaphors.
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It's wishful thinking, but an ideal operating system should...
- Run on my current hardware.
 - Require equal or less system memory than the previous OS.
 - Require equal or slightly more hard drive space (for the core functionality) than the previous OS.
 - Run as fast or faster than the previous OS on the same hardware.
 - Include the previous GUI skin as an optional install.
 - Back itself up. Automatically.
 - Not reduce any of my peripherals to paperweights.
 - Allow me to do more with my computer than the previous OS.
 - Display error messages a human without a computer science degree can understand.
 - Not reduce me to a quivering ball of cytoplasm when things go wrong.