Not having to tweak the config using the command line is sure a must-have for the general user. But they better let the option there. Because no matter which OS, something, somewhere at some time will need extra configuration that ain't available in GUIs. And in some cases, it's way faster to make your way trough the terminal rather than going deep into poorly designed configuration panels (which are present in
any OSes).
But to go back to the circle of hell up there

, "lack of commercial software and games" directly enters in there. Companies will not spend money on developing software on alternate operating systems like GNU/Linux because in their views (I generalize here, it's not the case of all the companies), there isn't a market for them to make money. But how do you want to make the market grow if it doesn't have support? It's a vicious circle! You never get out of it. It's not so much a question of usability here, give your mother a completely installed GNU/Linux system like you would have with a Windows system, show her where the buttons are and some "basic training", and it shouldn't be as difficult as making the transition to a Mac (and I speak with experience here

). It's not in the mechanics, it's in the software used. What are the OSS counterparts of what you usually find in Windows or Mac OS? Are there such counterparts? That is where it matters. It is possible to un entirely on non-commercial software, but it requires adaptation. And in some cases, due to the environment and other variables, it's just plain impossible.
So is it about the desktop not being user-friendly? 'Don't think so. But lack of commercial support? karajorma nails it right here.