This question has been bugging me for a while since GNOME 3. Nowadays in advanced countries, computer education is part of regular school curriculum. Children are taught what is CPU, what is graphics card, what is memory, how to turn on/off computer, how to behave on the net, how to use word processor, tabular calculator, presentation maker, sometimes even databases or easy programming. Unless in really rich school they are taught usually on something like Windows XP, they know the classical desktop. And they are confused by interfaces like Windows 7 and Gnome shell.
I am 24, my first desktop was Windows 3.11 (at school), then I grow up on Windows 95/98/2000 and Windows XP after finally switched to linux. I'm not that old to not be able to adapt, yet after years spent with linux I find the old Windows ineffective (just find something it the long non-categorized menu of theirs or try to run a few apps at once) and just recently I had to navigate Windows 7 with a touchpad. Let me tell you, it was a major pain in the ass. In my family everyone except me now uses Windows 7 on regular basis and they always complain about it. I haven't heard them ever complain about user interface when in Win XP or Gnome 2.
So why do the heck do we need the change that makes an assumption that presenting user with huge icons of every app installed instead of neat two-level categorized menu is more easy to use (it does not take a very bright person to tell that Movie Player would be categorized under Sound&Video)? Why do we need a change that makes an assumption that customization is a bad thing (I believe that different people have different needs and customizing a desktop to certain level can make it more effective to use for them)? Why do we need a change that effectively tells you that having more terminal windows open at once is a bad thing (yes, even though most common terminal emulators have tabs, I still find it more effective in certain cases to have three or four windows of terminal opened at once and nicely tilted on the desktop)? Why do we need a change that hides your favourite apps when not in "Activities" overview?
Who the heck is the modern desktop designed for? Honestly saying, not for me and not for anyone I know personally either.