Just got a chance to look at KDE 5 a bit (only through video and screenshots). I don't know why, but KDE just never appeals to me. It looks so basic and the arrangement of things always seems cluttered and cramped. Instead of making full use of screen space, their designs always seem to constrict things to small areas.
Their latest iteration of the K menu (I think it's called Kickoff now as opposed to Kicker) is a perfect example. You click on the K menu as you would a Windows 7 Start menu and you get a small menu in the left corner that has different contexts. You can select between Computer, Recent, Leave and a few other contexts so that the single menu changes. While this is not as messy as the ever expanding menus of MS Windows Start, it's also needlessly limited in use of space.
Contrast that with Gnome's "Activities" screen which takes up the entire screen (excellent use of screen space) or Unity's even better Dash (also uses the entire screen as a beautiful overlay) and you quickly see how they continue to cling to ageing launch paradigms.
Similarly, in one of the videos I saw a dialogue box appear which contained a scrolling list of many items. But the window was vertically compressed to only 1/8th the size of the list, so you are forced to scroll even though there is plenty of screen space to more sanely size the dialogue. I've seen this same behaviour in all incarnations of Windows since Win9x. This NEEDS to go.
You should NEVER have to scroll anything in a dialogue box. Scrolling in dialogue boxes is either caused by a poorly thought out set of options, or a poor window sizing policy. Admittedly, some of my prickliness in this arena might come from my having become accustomed to full screen everything in Xmonad and wmii. For me, that's the most efficient way to work because you don't need to fiddle about with scrolling lists or minimizing and maximizing. This is probably why I prefer Windows 8 Metro over anything MS has done desktop-wise.
At the end of the day, it comes down to learning about better ways to work with technology. Having used alternatives to Windows and Mac desktops, is akin to eating more expensive and harder to find but healthier foods. If you're forced to go back to eating out of vending machines and McDonald's you kind of feel it... and resent it.
Yes, this is opinion and not fact. But, what isn't opinion in the desktop wars? Especially now that the need for the desktop is going away with cloud apps. We're already at a point where no one really cares what desktop they use as long as there's a decent browser. THAT is as it should be for basic users.