Hardware interaction is the hardest part IMO.
My mom acquired a digital camera and an MP3 player from my younger brother, and without even so much as looking for a CD, she just plugged them both into her XP box and starting transferring files. She's 54 years old and knew enough to simply plug them in and see if they showed up in "My Computer".
How would you explain trying to install either of those on a Linux system? And then transfer files to them? Sure, there are those flavors of Linux out there that, with enough "snap-ins" on top, could make that work.
Contrary to the Apple marketing slogan, people use Windows because "It just works." Yes, we can all laugh and joke at security patches and lockups, but as their respective user and hardware markets expand, OSX and Linux are starting to experience the exact same issues. It supports pretty much every piece of PC hardware manufactured on this planet for the last 10 years (Vista has finally broken that, which I think is actually a good thing...), and supports apps even further back.
You click on something, it installs, you use it, the end. Until Linux is that simple, it won't have the marketshare.
Humorously, OSX is starting to pick up -- I think they're at 6% now? OSX is a great piece of software, but I think Apple's marketing is going to ultimately kill them if they continue down the same road. As more and more people start using the Mac platform, it's going to become obvious that it has it's own problems just like Microsoft does.