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It would have been good if you had said at the beginning that you are speaking to home users. The reason why Vista is turning out to be such a cluster is backward compatibility. OS X is not concerned about old software. If you buy a new Mac, don't try to run your OS 9 software.
For years, businesses running Windows have contracted with development companies to build applications specific to that company. As the Windows OS iterations occurred, they had to be able run those old developed apps. Apple has not been concerned about that. Since Macs are primarily for home users and graphics professionals, the Mac appdev community is really small. Backward compatibility is not much of an issue.