I would say it has to do with then open source movement attracting more coders then artists. Also remember that Apple spends a lot of money on their user interface and they still take criticism.
You know, it's just a different approach. The differences between particular themes are inevitable, as there isn't any sort of common guidelines, AFAIK.
For me, Linux themes are completely integral, I've yet to experience any sort of feeling of awkwardness.
Well, the OSX-like theme has unaligned text placement but most of the others on the page you linked looks fine to me?
Also, It's not too hard to fix those things. As @nemo stated, there are "way" more coders than artists in free software world. If you make suggestions as an artist to a theme maker, they will probably be incorporated in the next release. That's how free and open source software world works.
The quality of the themes also vary.. It shouldn't be that hard to find a theme that's "horribly wrong" because of its basic choices and priorities. Personally, I like very compressed themes. I even used the Enlightenment window manager, known for it's eye-candy, to get rid of window decorations completely. So I might choose a violently cramped theme and be very happy, despite the odd glitches that make you wig out.
I'd instead say that Gtk/Gnome is really the reason you are looking at a GNU/Linux desktop at all. The stuff that went before it was truly heinous all the time! (No offence to the creators of XAW, but they probably didn't have "lickable" as a goal...)
I'm using KDE with one of default themes (Plastic) and I'm yet to see anything awkward there.
I also use Gtk2 applications and still haven't seen anything awkward there. You should know that using any theme that is not officially supported by Gnome/KDE can have bugs.
However, all this can be very subjective. For example, I find some of Windows XP UI to be awkward and/or just plain ugly. Different people have different tastes and habits. Maybe it's because I used Windows98, then Linux, then Windows XP, so everything is awkward. And you are probably coming from the other direction.
The problem is twofold:
Additionally, theming is evil and should never have been implemented. But thats just my IHMO, it totally ruins consistency.
The entire linux desktop doesn't have the visual integrity of OSX for instance, and I wonder why. If there is any example of a nice integrated Linux environment, please please please show me, I really WANT to use Linux.
(and I know, there's QT, and other managers like KDE etc. I noticed the same thing, so it probably isn't GTK or Gnome alone)
You basically just answered your own question - Mac OS X has one desktop environment (centered around Cocoa), which has a lot of very good programmers and designers spending much time designing, discussing and implementing it.
With "Linux", there's not only different window managers (Gnome, KDE, Flux/OpenBox, Enlightenment, etc etc), there's different UI toolkits (QT, GTK+, Tk, wx, etc etc), different versions (in common use), and countless different ways of using/implementing them.. Compare List of (mainly) Linux GUI toolkits compared to for Macintosh (all of those, bar Cocoa, are for old Mac OS versions).
There's many good designers and programmers working on GUI toolkits, but they are spread over so many different projects. Even if they somehow were put onto a single project (Say, "wxQtGnome"), there would still be countless applications that use a different framework, or even a different version of one.. Then you still have the problem with inconsistency. Again, compared to Mac OS X, where if you have a GUI application, chances are it will be using Cocoa (which has a decent UI creation tool, which helps developers layout applications according to Apples Human UI Guidelines)..
As for a nice, integrated linux desktop environment - I would say the default Ubuntu installation is probably the closest to this. A decent theme, and all the applications fit fairly well together.
As for the example picture, maybe you could post similar pictures for GUIs that you do like (Mac OS, Window...)? That might make it easier to compare the GUIs and see subtle differences.
Anyway, I think the preview images on art.gnome.org are not representative for the real look. If you want to compare GTK+ GUIs with other toolkits, I suggest to look at screenshots from real apps. For example, Ubuntu with default theme should be quite representative.
Btw. the Gnome HIG (Human Interface Guidelines) have a chapter on layout and spacing: http://library.gnome.org/devel/hig-book/stable/design-window.html.en .
I don't think GTK+ itself is responsible for the "awkward look" (which is your subjective opinion!) of Linux applications.
In fact, GTK+ is very flexible with regards to the possibility to apply themes and even completely different rendering engines for the widgets (see the popular Murrine engine for example). It's much more flexible than the native GUI libraries of Windows and Mac OS X.
You can find lots of GNOME themes on for example gnome-look.org. There are even GNOME themes that make your GNOME desktop look almost exactly like Windows Vista or Mac OS X (I don't understand why anyone would want that, but that's a personal thing...).
Canonical, the company behind Ubuntu, has been saying for some time that they're hiring professional designers to create a very sleek and professionally designed desktop theme for some future version of Ubuntu.