For a start the default Windows XP configuration sets up the account with administrator rights while every Linux distibtion I know of forces you to have a separate user account.
So? What permissions are required to place an executable file called evildaemon.bin in ~/.malware/ and set it to start up when KDE loads?
Linux is far cheaper to maintain than Windows - lower total cost of ownership.
In order for the masses to use it, there will have to be seamless auto-update that doesn't involve the command line, and lasts for more than 18 months per version. Name a distro that supports this without having to pay. If there were 100,000 million desktop Linux users, the existing free update solutions that certain linux vendor's support just wouldn't work, because of the enormous costs involved.
Windows is far harder to modify and customize.
But does the average user give a shit about this?
Windows requires 3rd party software to run well, while Linux comes with everything you need.
And you don't think that would change if
everyone switched to linux?