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What is so wrong with Windows? Seriously. It's not perfect, but I'm yet to find an OS that is.
The big one - forking is far too slow. Others - you can't use select(..) on files. Nasty file-locking model. No good remote filesystem solution out of the box (CIFS is slow and unstable) and the third party solutions are nasty. No native workspaces and the third party attempts are nasty. You can't reflect against the system API (all current mainstream OSs suffer from this). The API is particularly thick with legacy o…
OS X did not have workspaces at all for many years, and the two different versions of it they have introduced are kind of lame in the details of how they work. (Specifically, Command-Tab cycles among all open apps, not just the ones in the current workspace, as is done on Linux, and there is no other quick way to switch to a different app in the same workspace -- Mission Control not qualifying as quick -- with the result that I sometimes make a point to put two windows that "belong together" in separate workspaces just so I have a quick way to switch between them.