One of the famous quotes about the X Windows System is this one:

Sometimes when you fill a vacuum, it still sucks. - Rob Pike

The first thing to say here is that it is not quite the case that X filled a vacuum and so succeeded by default. Certainly X was the first cross-Unix window system, but Unix workstations existed before X, so of course various Unix vendors had to come up with GUI environments and window systems for them like SunView. However, people do not speak entirely favourably about those systems.

Early Unix window systems were not designed by idiots (despite what some commentary might say); they were designed by dedicated, smart engineers who were doing the best job that they could at the time and on the machines that they had. It's just that people didn't yet know enough about window systems to see what was a good or a bad idea, what sort of APIs worked, and so on, and it didn't help that early machines and Unixes were very limited. Mistakes were made; indeed, mistakes were inevitable. So the early window systems were rather far from ideal and once the dust settled, not entirely appealing.

This is part of the vacuum that X filled (although not all of it). X came along at the right time to learn from prior experience (and evolved a couple of times without having to worry much about backwards compatibility issues), and the result was, well, quite New Jersey. It worked, and through at least the early 1990s, when it didn't work people bashed on it until it did. Generally it even worked better than its competition at the time or delivered compelling additional features, or both (although this is not the only reason it succeeded so well).

At the same time, X has never been elegant. In this it partakes of the spirit of (V7) Unix; not the side that gave us the clean Unix design ideas, but the side that gave us a kernel where active processes were just put in a fixed-size array (that was walked with for loops):

struct proc proc[NPROC];

Why not? It worked well enough, and V7 was a New Jersey approach work in progress.

That X is not elegant or simple or very Unixy is what Rob Pike meant when he said that it sucked. X is a generally unattractive beast under the hood, full of protocol and API complexity, with various messy features and any number of omissions of things that people would really like to be covered (some of which were later sort of added). It's not as policy-free as it claims to be, and anyways being policy-free is ducking various important issues (especially if you want disparate clients to genuinely interoperate). But X did in a sense fill a vacuum and more broadly it definitely filled a need, and doing so was (and is) important. The logic of X existing is inorexable and it's a clear improvement over what came before. For all of X's warts, no one really wants to go back to SunView or the other pre-X options.

These days, my own reaction to X's warts is basically to shrug. Given that it's the best option I have in practice, well, it works well enough; in fact, it generally works pretty great. I'm aware that a great deal of sweat and irritation is being expended behind the scenes to make that happen and every so often I stub my toe on a rough edge of modern X, but for the most part I can ignore all of that. I would like it to be more elegant (or to be magically replaced by something that was), but it's not something I'm passionate about. I am passionate about not going back to a long catalog of not-as-nice window systems that I have used in the past, sometimes out of necessity. And yes, that even includes cool ones like the Blit.

(I'm not convinced we know enough to do a window system design that's significantly more elegant than X and still has X's virtues, including working well across the network. I guess Wayland may give us a test of this. There was NeWS, always the great white hope of Unix window systems, but I think it had the wrong core design and anyways who knows what would have happened to its initial elegance after ten or twenty years of being adapted to the harsh realities of general use.)

PS: Yes, X is not supposed to be called 'X Windows'. Sorry, that ship sailed years ago; in practice, 'X Windows' is a perfectly widely accepted name for it in situations where you don't feel like spelling out the full name but 'X' or 'X11' is too short.