X is an outdated architecture-independent system for remote graphical user interfaces and input device capabilities. Each terminal has the ability to interact with the display with any type of user input device.

 The Unix-Haters Handbook (1994)  (here is a local copy so you can read it online ) devoted a full chapter to the problems of X. Why X Is Not Our Ideal Window System (1990) by Gajewska, Manasse and McCormack detailed problems in the protocol with recommendations for improvement. The lack of design guidelines in X has resulted in several vastly different interfaces, and in applications that have not always worked well together. The Inter-Client Communication Conventions Manual (ICCCM), a specification for client interoperability, has a reputation for being difficult to implement correctly. Further standards efforts such as Motif and CDE did not alleviate problems. Systems built upon X may have accessibility issues that make utilization of a computer difficult.

Incidentally,  The Unix-Haters Handbook was just an expression of the fact that Unix is and was one of the most popular and best-known operating systems in the world. Similar unfriendly things could be said about all operating systems in the world but Unix was widely used, especially in the academic world, and was criticized accordingly. So  The Unix-Haters Handbook should be seen less as a criticism of Unix, but rather as a suggestion on how to write an almost 'perfect' system. Especially in view of the fact that the criticized errors are easy to correct and Unix  (rewritten from scratch modern Unix or even on the basis of a (modified) free Unix like Linux, BSD, Qnx,...) can be easily adapted to today's hardware and conditions. There is so much more know-how today accessible on file systems and all other crucial parts of an OS, that it should really be no big task to accomplish this.

   And finally this remark: a graphical user interface (GUI)  is absolutely no rocket science, but almost trivial with modern hardware if you really understand its function (terminals are obsolete, terminal servers are obsolete, display servers are obsolete, "remote desktop" servers are obsolete, "remote graphical user interfaces" are obsolete, ...)  and how it fits in an operating system and don't overload it with useless functions. Close integration into the system is necessary to avoid wasting any cpu time. While all commercially available systems have long since taken this step, Unix is the only one that still has a (problematic) "remote graphical user interface" despite all the criticism over the years.

Wayland

   Alternative to X11: Wayland, the next-generation display server, a modern, secure, and more straightforward windowing system. Wayland was startet by Kristian Hogsberg as a personal project in 2008. As an X.Org developer, Kristian knew about the defiencies of X11. It promises to reduce the communication overhead of X11. But read:  Wayland Protocol and Model of Operation.  How Wayland differs, you can read here: Wayland Architecture. Increased security is the main advantage over X11.

 Most video games and graphics-intensive applications for Linux are still written for X11. Also, many closed-source graphics drivers (eg for NVIDIA GPUs) do not yet offer complete support for Wayland.  And Wayland still has issues (read also here). Nvidia just released a new driver that fixes a bunch of these issues.

But Wayland is more secure than X11 which is prone  to  “keylogging”.  Any program in the background can read what’s happening with other windows open in  X11. In Wayland this can't happen, as each program works independently.

Some links for Wayland news: here and here and here and here and here

Some Linux desktop environments:

GNOME is the most popular desktop environment for Linux

KDE read:  KDE has managed to beat XFCE as one of the lightest desktop environments

Mate is based on GNOME 2 and was  developed for the users who were disappointed with the latest iteration of GNOME shell — GNOME 3.

Cinnamon is a fork of GNOME 3

Budgie desktop has been developed by the Solus project, they utilized GNOME technologies such as GTK to develop  Budgie

LXQt is a lightweight Qt desktop environment. LXQt is indeed a lightweight desktop environment while providing a better user experience than LXDE

Xfce is one of the most lightweight desktop environments at the moment.

Deepin  may offer a macOS-ish interface

Trinity Trinitys primary goal was retaining the function and form of traditional desktop computers

Lumina is designed to have a small footprint, giving your system the best performance possible. Lumina is extremely minimal and can be used on systems with as little as 1GB of memory. It is very self-contained and does not require any particular utilities or libraries aside from very small handful. Lumina is designed around the concept of complete modularity. Your applications are completely independant from the desktop itself and can be added/removed at will without loss of functionality.

Xenix  (outdated)
was a drevivative of UNIX by   Microsoft.  ( UNIX Ver. 7 for 16-Bit-Mikros, later MS- SCO )
 
Xenix was Unix -- or at least one flavor of it.  In the late 70's,
  Microsoft licensed the Unix sources from AT&T and ported them to
   a number of platforms.  In those days, AT&T would license the Unix
    software but not the Unix name, thus each company had to invent
    their own name.  Microsoft picked Xenix.  Microsoft did not sell
    Xenix to end users.  Instead, they licensed the software to OEMs
    (Intel, Tandy, Altos, SCO, etc.) who provided a finished end-user
    package.  Microsoft no longer supports Xenix, and in fact never
    even offered a 286 or 386 version.

    Several Unix implementations for the PC architecture have been
    tried with varying levels of success.  SCO Xenix for the PC/XT
    was one.  Nearly all of the PC/XT implementations were clunkers,
    because the machine lacked the hardware necessary for robust Unix
    operations.  The PC/AT offered hardware memory protection, and
    SCO Xenix/286 took advantage of it.  SCO Xenix/386 added demand
    paged virtual memory.  These added features made multiuser PCs
    viable, and SCO Xenix popular.

    SCO Xenix starts with a Unix System III base, throws in several
    Berkeley enhancements, and adds features to obtain conformance to
    the System V Interface Definition (SVID).  Today, the bulk of the
    code is from System V.  Xenix/386 even has capabilities to execute
    Unix programs.  It differs, however, in many of the SVID `optional'
    areas people tend to expect of a full System V.  SCO Xenix lacks
    a real `inittab', for example.  You need to go to a real System V,
    such as SCO Unix, for all these features.  

x-kernel (University of Arizona)
The x-kernel is an object-based framework for implementing network protocols. It defines an interface that protocols use to invoke operations on one another (i.e., to send a message to and receive a message from an adjacent protocol) and a collection of libraries for manipulating messages, participant addresses, events, associative memory tables (maps), threads, and so on.

X Display Manager

Cygwin/X (X-Windows)

Xming

XQuartz (macOS-X)

Rio (Software) x-window succesor

All about OSs