This is a great article by Neal Stephenson about the evolution of the Operation System as a marketable product. His roots in computing go all the way back to punch card mainframe systems, and moved to Mac's and then to BeOS (which I'm actually not at all familiar with). Its really a fascinating insight into how the current OS market evolved and where its possibly going. While you don't need much computer background to grasp his points, they are firmly rooted in computing examples.
As much as I want to ditch Windows altogether (I'm currently writing this on a dual-booting laptop with Windows 2000 and Gentoo Linux), I find even for me the critical apps aren't there for linux... yet. I don't doubt they will come. Once a few businesses and governments take the plunge and switch, the companies that make these critical applications will suddenly realize there is a huge market for their products (even in an arena dominated by Open Source). Open source can do a lot, but things like accounting software (Quicken) and imaging (Photoshop/Paint Shop Pro), etc. still on have Windows (and/or Mac) offerings. But do believe in the next five years, you will see these major programs ported to Linux.