While I haven't exactly done the research but it would appear that the majority of those who purchased Vista are "power users" to some degree. Which is ironic considering how Microsoft emphasized the user-friendliness of Vista when it comes to the casual user.
Of course, like Philly says, Vista will eventually end up on most PCs simply because it is bundled by default with most new PCs sold. I think the biggest factor in the not-so-overwhelming reception of Vista is, and I doubt it would come to anyone as a surprise, the price.
Let's face it, the only thing that the average Joe sees new in Vista is the interface (it's also the only thing that MS advertisements for Vista advertise), and who's going to shell half a G for a new interface. Besides, for the average user, XP is good enough and that's all they average user really wants. Also add to that the hardware upgrade factor, as almost every article about Vista points out that unless you've bought/upgrade a system within the last year you'll need to upgrade at least one or two components (RAM and Graphics Card usually) to take full advantage of Vista.
