KDE4 overtaking GNOME in terms of usability?
If it hasn't happened already it seems poised to happen sooner or later, albeit usability is to some extent in the eyes of the beholder. My basis of this expectation is personal experience of KDE4 improvements thus far and observation of future trends for both platforms.
Perhaps the biggest change visible to the user is making everything into a widget, but the default and typical configuration of those widgets still resembles a typical paradigm, and that seems to sit well with the users. At least it does for me. I'm increasingly finding it more natural to have a single panel at the bottom with all of the stuff I need immediately available than to alternate between upper and lower panel.
GNOME 3.0 on the other hand promises to completely do away with the typical main menu currently resembling Mac OS X and replace it by something that probably wont be very familiar to neither Windows nor OS X users. Upon pressing of the "activity" button an entire screen transforms. It's not just another menu, it's a whole new "mode" of operation. It's yet to be seen how are users to respond to this as they begin to test it in day-to-day use, but I am somewhat skeptical and tend to side with some of the Bruce Byfield's commentary. Furthermore, as he points out, it doesn't appear that the GNOME Shell is getting enough opportunity to be tested by users for valuable feedback. This increases chances of GNOME 3.0 becoming a similar scenario to KDE4.0.
In short, as KDE4.0 rapidly becomes more and more comfortable in its new skin, improving significantly with each release and unfolding it's potential like a caterpillar turning into a butterfly GNOME is facing a major shift with an interface change that seems to go beyond what KDE4 underwent.
Speaking of usability improvements there are a few KDE4 features which are rapidly becoming a kind of a "killer app" for KDE, some of which have existed even in previous KDE generations and some of which have been introduced by KDE4.
KDE is known to allow the user to adjust far more options than GNOME (which has been known to hide options for the sake of usability). KDE3 therefore often ended up seeming complex and overwhelming yet KDE4 appears to be succeeding in blending this flexibility with ease of use as is perhaps best testified by its new control panel, but also to some degree the various configuration dialog styles across the system.
Take for instance the dialog for configuring toolbars in dolphin (and some other KDE4 applications such as Dragon Player).
A whole slew of available toolbar actions are available on the left, but a typical set of what most users may want is already selected on the right. To add new buttons all you need to do is select the desired action and drop it to the desired position for the toolbar, and if you prefer you can also use the arrow buttons. It's pretty simple, yet it didn't require hiding options. So at the same time KDE4 here succeeds at tremendously empowering the user and keeping things simple.
It's this sort of thing why I am beginning to love and prefer KDE4. In GNOME, things sure are simple, but that goes at the expense of flexibility. KDE4 gives me both without the extra flexibility feeling like too big a burden.
This theme seems fundamental to what may thrust KDE4 towards overtaking GNOME in terms of usability. Consider for instance KDE's traditional ability to choose what to do with the file you're drag and dropping. The menu appears on the destination, right when you need to make a choice. I've always envied this about KDE when using GNOME.
Again, giving me options without feeling like a burden. They're there right when I actually need them. I know countless of times that I wanted to make a bunch of shortcuts on the desktop in GNOME and always needed to make a shortcut (right click > make link) in the current directory and then move it to the directory I wanted them in because GNOME seems to offer no way to do this in a single operation. There might be tweaks that allow doing that, but they're not enabled by default and most people don't know how to enable them.
Not only that, but GNOME insists on naming the short cuts "A link to x" rather than just "x" so after moving the links I also have to rename them. Furthermore, if the short cut is on the other partition, I also have to delete the duplicate because in partition-to-partition operations GNOME's default operation is to copy, rather than move. Yet in KDE a single menu popping up with choices right when I need them deals away with all of these issues conveniently.
Another good example of flexibility that aids usability is KDE4's ability to select and deselect files and folders by just clicking on "+" or "-" emblems. This allows me to use one-click launch of folders and files without it getting in the way of selections. Without this feature the only way to select a single file without launching it as well is to draw a rectangle over it or press ctrl. Now I can simply click "+". No ctrl or rectangles needed. Single-click operation suddenly becomes a no-brainer as now there are no disadvantages to it. Awesome.
I could go on and on. There are many interesting details. Just think of krunner, kick off, options for window manager buttons, wallpaper modes etc. In any case, whereas GNOME so far made sacrifices to flexibility and power in order to improve usability KDE4 is succeeding in having its cake and eating it too. They merge flexibility with ingenious design to create a desktop environment in which having lots of options to choose from doesn't impair the ease of use and in some cases as seen above actually aids it. The advantage over GNOME's approach seems clear to me. I'd rather have both power and ease, then just ease and a sense of lack.
I only wish Kubuntu was given more attention in the Ubuntu development process. It still does to some extent feel like a second class citizen, yet I think it is more deserving of spotlight than ever before.