Canonical actually spent money on user interface testing. I can find some activity for Gnome shell but I'm having problems digging up any solid testing for KDE.
Usability people once came up with a brilliant observation about "spatial" and someone used that to argue that everytime you clicked on a folder in Gnome it should open in a new window.
Of course this made desktop a mess.
Many usability people also claim that it is good to have the menus attached to the top of the screen instead of attached to the window it affects.
Software recommended by usability people often have menus hidden away behind "gear" or "hamburger" icons.
I am not a usability expert but often I feel software is worse off after the ux people have had a say.
In particular it seems for many of them the goal is to mimic Mac OS X.
Remember that you are in the top decile of computer users if not in the 99th percentile.
Unity (Canonical's fork of Gnome Shell) perpetrates neither the 'spatial' filer nor the hamburger thing. Gnome Shell's Nautilus no longer uses 'spatial' presentation either - I recollect that Acorn RISC-OS used that metaphor.
I think that menus at top of screen made perfect sense in the days of screens with resolutions of 512 x 342 - I used a Mac LC - and on netbooks which is where Unity came from after all. On a large monitor they make a lot less sense I agree - huge trek to the top of the screen to click on stuff and the danger of a misplaced click on the way changing the menus.
I stopped using Unity for ages because of the way it broke the 'ALT-F' style mnemonics (Alt-IOF to insert a typeset equation in LibreOffice &c) but things have improved recently.
I find myself in the strange position of actually rather liking Gnome shell from Debian Wheezy onward (3.8)
Canonical actually spent money on user interface testing. I can find some activity for Gnome shell but I'm having problems digging up any solid testing for KDE.