So to you Quartz is just a Compositor.
In general, the term Quartz or Quartz technologies can refer to almost every part of the Mac OS X graphics model from the rendering layer down to the compositor. In this use, the term covers Core 2d, Core Image, PDF Kit, Core Animation and Core Video as well. https://developer.apple.com/referencelibrary/GraphicsImaging/idxQuartz-date.html

OTOH, X is "just a protocol".

Yes but a lot more still need to be done.
Keith identified a lot of X's problems can't be solved at X's level and should be fixed at "above xlib" or whatever. Some of them freedesktop.org is working on. some were lost. e.g. color management system.

Keith also talk about compressed image transport.
Today, to display a jpeg file, an x client (eg firefox) decompress the jpeg and send the decompressed image to remoteX. If X is used oven ssh, the image will get jpeg decompress and compressed by ssh and decompressed again at remoteX...
While XIE is dead, I'm sure Google Chrome OS will get it right.

...

Quartz use a PDF like vector object model. With Quartz Extreme the whole thing is GLSL running on GPU. X itself don't have anything like that and have to use addon like Cairo. Since most X apps don;t use Cairo, even if there is a GPU accelerated Cairo, most apps don;t get any speed up.

Don;t know what's wrong with X? Read "Papers and Talks by Keith Packard":
http://keithp.com/~keithp/talks/

remote x need high bandwidth and low latency networking. Especially round trip issues. So it works OK on LAN but suck on the internet.
Most users use vnc and or nx anyway. Anything is faster than remote x on internet. If X did it right the first time there is no reason anyone would ever need nx.
(NX is basically a roundabout way to get around X's problems/bugs/limitations to make it works in real live situation. It's written by people really use X and know all the low-level problems. Amount other thing X protocol is very verbose, often send redundant and repetitive parameters over the net. NX try to cache/compress/delete those parameters to save bandwidth)
BTW, Google just release Neatx, an Open Source NX Server
http://google-opensource.blogspot.com/2009/07/releasing-neatx-open-source-nx-servier.html

remote X's #1 problem is round trip delay. To solve that problem, browser use client-side javascript. So X need some sort of server-side scripting. I'm sure Google Chrome solve that problem already!

Bingo. OSX did it.
overall X's #1 problem is toolkit. Anyone still use Xt? On X everyone write their own toolkits. "radical redesign of all existing GUI toolkits" is easy to do if there is only one standard toolkit.
Similarly X have multiple window manager, so any real improvement require radical redesign of all existing window managers.

pH