I highly recommend the talk by Daniel Stone who used to be a core X.org developer. He explains it quite well how X is used currently, and why it has problems and why they are considered so hard to fix that Wayland (and Mir) was created.
https://www.youtube.com/watch?v=RIctzAQOe44 One interesting point he made was the X server is no longer network transparent like it used to be. It is network capable now but when used in that way (ssh forwarding), it's essentially done in the same way as VNC, but more poorly because of the way X11 is architected. -- https://mail.python.org/mailman/listinfo/python-list
