I was just reading this article over at ZD Blogs. Adrian submits that the "killer app" argument is dead, i.e. you don't get Linux or Apple adoption because there's no "killer app" on that platform that people need like there is on Windows. His argument is that the "killer app" is really a web browser these days, so the argument is outdated. It's a good read and I recommend it.
I dispute his findings though. The web browser is a commodity, not the "killer app." It's become as basic as the drivers that make your video card work. Without it, a computer isn't a computer.
He also takes the stance that most people don't need Photoshop or Autocad, inferring that you may not need everything that Windows offers. What he neglects is the pro grade software available for Linux and Mac that do as good or better of a job than their Windows equivalent in these niche markets.
Case in point: I have a dual-boot Windows XP / Ubuntu Linux system at home. For photo work I actually prefer to boot into Linux. I use LightZone and Gimp in Linux for all my photo work. I do my HD digital home video editing with Cinelerra. I do my desktop publishing with OpenOffice or Scribus.
The new "killer app" is multimedia, and with Windows' DRM features that prevent you from even using your own MP3's on your own MP3 player it just makes Windows obsolete.
Have you ever tried to use Rhapsody? Yeah, I tried once. I went back to my K-Pex Linux based MP3 and video player.