A few years ago I found a book on Motif user interface programming in a used bookstore, and started leafing through it. The entire book had exactly ONE picture. ONE. And it was some architectural diagram describing X, depicting a network of terminals and servers and happy little cloud things. The rest of the book (asymptotically, all of the book) described APIs and widgets and protocols and whatnot, all without the benefit of showing what the user would be interacting with.

That someone could write a book on UI programming and someone else would publish a book on UI programming without any actual depictions of UI is pretty much all I need to know about X and its community (although I know a lot more, and my actual exposure to X goes back to early versions that came on 9-track tapes directly from MIT and supported mostly just frame buffers on DEC Vaxen).

(For years -- and this may still be true -- the software to manage the equipment in Comcast's head end datacenters was controlled by some X-based UI. Now, I ain't gonna say "Them folks surely deserve it, because they done me wrong" but that miserable train-wreck of a system goes a long way towards explaining why it's often hard for Comcast to fix their stuff. I have un-fond memories of taking down whole QAMs because I was fool enough to click some check boxes in a dialog in the wrong order...)

You can write hideous UI in just about anything, but X seems to have a special place in the ecosystem.