- they will lool for the desktop for ever, not understanding it's gone;
- will struggle with the dock, hidding, not opening new windows when expected, not showing when several windows are opened, etc;
- will have a painful wifi setup (the list of SSID don't refresh unless you close it, then go 2 menus down to restart it)
- will have a painful bluetooth setup (the menu doesn't even appear if it's off)
- will be confused when trying to minimised two windows to put side by side now that the icon is gone;
- will try to look for the button to put the laptop to sleep and look at you with the eye of a sad puppy;
- will see a lot a spaces wasted compared to unity because of the windows bar being back and huge;
- will look for their app tray icons (now at the bottom left);
- will activate the hot corner by mistake.
I use myself 15 addons to make gnome-shell usable, and you can't expect the average user to do that.
Gnome shell is still experimental. Not from a technical point of view, but from an ergonomic point of view.