I've been using Windows as a super user since around 1988...! I love WinXP more than any other OS ever made. I worked for MS for 5 years. And yet, today my long-term goal is to raise my kids on Linux.
But ... when switching one computer to Linux, any problems cause friction in my family: typical discussion points are
Why do I have to be different; what's the point; but it worked just fine before.
Bottom line: The most important aspect is (lack of) seamless integration into a Windows-based home environment. That's what sent me back to Windows every time.
During the past decade I've had several attempts at switching to Linux. My first attempt was Red Hat and it was just ridiculous; I felt like a stone age man in a spaceship. It's fine that distros have advanced uses, but those just aren't for novices.
With my more recent attempts, with Ubuntu and PCLinuxOS, there were two major problems that always sent me back to my beloved WinXP: hardware support (Nvidia and audio); and poor integration into a Windows-based home network (file shares). I am aware that I can't expect exactly the same software; only other software for the same purpose. I think many people don't really understand that small difference, and they become disappointed.
On every computer ever used, I never got the Nvidia card to work with my monitor. I only found out this week(!) that the problem wasn't Nvidia itself but rather that my monitor didn't send proper EDID signals back to the computer. Once I learned to define the monitor's specs in xorg.conf, the issue was solved. Oddly, I never had that problem on Windows (with the same monitor) so I had no clue that it was the culprit.
Well, it's a lesson learned, but it cost an unreasonable amount of forum posts, failed attempts, frustration, and unhappiness. I think it's significant to observe that my install only succeeded after the launch of askubuntu.com...
This is still an outstanding issue - my desktop computer is in a pure Windows environment, and it's the printing host for the family. It's essential that all the home's computers can browse each other's (Windows) file shares, and that they can print. If I can't make file&print sharing work, I am forced back to Windows - again. So this attempt at Ubuntu has not fully won me over yet.
Setting up a share in Ubuntu that a Windows computer can read&write is very complicated to figure out for a Windows user. Also, getting access to Windows shares from Ubuntu is just as complicated. I always run into problems with missing user accounts and access permissions.
Ideally, everything will end up working so well that I would one day be able to convince my wife to switch to Linux too, but that day is still very far off.
I would also like to switch my media center pc to Linux, but it's not just a player, it also has TV tuners and I have never ever been able to get them to work with any distro - but Windows just works. Digital, analog, EPG, everything; using only a for-dummies onscreen setup wizard. I need that kind of quality on Linux before the media center will move from Windows, and I just don't see that coming.
Ubuntu has made a tremendous difference for novices in the Linux world. Before Ubuntu, there was nothing that was seriously approachable for novices. With the last 2-3 years' releases of Ubuntu, things have become so fantastic that reasonably experienced pc users can have a go at Ubuntu and not crash&burn as I did with Red Hat. That's remarkable! While it's not actually mainstream just yet, the past few years indicate that "mainstream" is a realistic goal not many years away. Just look at how awesome the Software Center is: search for anything and you'll find not one but several choices, each of which is free and installs in seconds, no reboot needed. Wow!