It seems hard to believe now, but some time in a distant past, Windows and X
Window were very close competitors and proponents of both windowing systems
would spend hours debating on the virtues of each.

Depending how old you are, you might never even have heard of X Window, so
allow me a brief introduction to this technology.  X Window was developed
at the MIT by Bob Scheifler.  The development pretty much started when
bitmap screens were emerging and that a handful of developers decided that UNIX
workstations should have a graphic user interface.

X Window is low-level and powerful at the same time.  It defines all the
standard basic operations you would expect from a windowing API (drawing shapes,
handling colors and fonts) but nothing else.  Concepts such as buttons,
menus and other widgets had to be developed separately (this is what other
toolkits such as Motif or Athena provided, but later). 

X Window also supported a very intriguing and quite innovative idea at the
time:  remote displays.  In short, the whole API was basically
"headless", to use the Java terminology.  The API assumed two very distinct
parts:  client and server, allowing an X server to display window on any
client workstation.

X Window’s flexibility was also its downfall.  By virtually allowing any
kind of window interface to be implemented, it created a market that started
fragmented and never recovered.  Trust me when I say that it was almost
impossible for even coworkers to use each other’s workstation without being
utterly confused by the various key remappings, window shapes and mouse button
bindings that each user had configured.

Dave
captured this phenomenon extremely well in his short review of Eric Raymond’s
book "The art of UNIX programming":

So, by providing policy, the designers of Windows and Mac interfaces have
provided their end-users with a consistent look and feel, and a base set of
application behaviors. By instead focusing on mechanism and ignoring policy, the
designers of X allowed developers to experiment, but gave the users of X
applications a very inconsistent interface experience. Arguing one approach is
better than the other is pretty pointless: they