Can't comment on KDE because I never liked them, but Ubuntu/Gnome has progressed quite nicely since 2005 (my first "Linux year"). In fact my opinion is 180 degree opposite of yours: 2 years ago I switched to OSX and couldn't tolerate how dumb and unusable the UI felt: I waited a year hoping to adjust but surrendered and went back to Gnome.
I've given up on arguing about GUIs though: it all comes down to individual preferences, oftentimes in coffee shops and offices I look at people and notice how bizarre some of their UI habits are, and I'm sure others can say this about me.
I think the thing that especially appeals to me about the OS X and W7 environments is that they're not an eye sore (warm colors & effects, not kludgy looking), and they have quick access to system-wide search that works (Spotlight and Windows Search, respectively). These are my rather minimalist needs.
I have a hard time coming up with an analogy, but when I open windows on one of the live CDs, I feel like there's a gnome (no pun intended) in the background grabbing and stapling up shoddy UI mockups on the inside of my LCD.
It all comes down to tastes, as you say, though. I used to be a MOTIF and IceWM guy, so mine have definitely changed. I can't imagine "most people" would like the state of the Linux UI, though.
Having used KDE (since it was an ugly Windows 95 clone, anyone remember those initial efforts?) and GNOME from their births, not much has really seemed to change. Meanwhile, the UI paradigms have shifted significantly, in the rest of the computing world.
On the surface perhaps, but the beauty of Linux is that what you see is not what you can get. There are plenty of cool UI paradigms available just an apt-get away.
For example, I use Ubuntu, but I auto-hide the taskbar and use Gnome-Do to access everything. Terminal is available via Guake drop down. Windows are arranged in workspaces on the surfaces of a 3D rotating cube. Mouse is 100% optional for controlling windows and OS. And if I wanted I could exchange all that with a tiling WM like Xmonad or Awesome (though I find Compiz Grid + Guake equally efficient).
Now, I haven't used Windows since my Win 7 eval copy expired last February, or OS X since the last time I walked into an Apple Store, but I don't see that their UI paradigms have shifted significantly at all. It's still basically the same mouse-controlled window systems, perhaps with a few new bells and whistles like desktop search, GPU-enhanced eye candy, and optimized performance thrown in. But none of those change the usage paradigm as far as I can tell.
I've given up on arguing about GUIs though: it all comes down to individual preferences, oftentimes in coffee shops and offices I look at people and notice how bizarre some of their UI habits are, and I'm sure others can say this about me.
Here's my take on Gnove-vs-others, and a good counter argument made in the comments: http://kontsevoy.blogspot.com/2008/11/ubuntu-8.html