When Apple introduced OS X, it chose to use its own proprietary GUI instead of X11. This provoked some criticism from the community. Mike Paquette, who did much of the design work on the system, replied, saying that they could have used X, but if they had they would have had to extend it in so many ways it would have been almost unrecognizable. Since then, Keith Packard and a few others have done exactly that.

The X Window System is conceptually very simple. It provides a mechanism for assigning a region of the screen onto which an application can draw (with a clipping region to prevent it from drawing over other applications’ windows), some inter-window communication systems, and very little else.

As a result, X earns an entire chapter in the UNIX Haters’ Handbook because it has lead to a large number of mostly incompatible extensions. Everyone adds their own widget sets on top of X, and even extended the core protocol in different ways.

At one point, there were three different extensions for providing support to screensavers, for example. At the height of the UNIX wars, each vendor was adding their own extensions to the reference implementation, causing interoperability headaches for everyone. This has largely been resolved now, since most people use the x.org X server.

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