Face it - Elvis is dead and both Vista and Leopard failed to make grab the hearts of users in the way that their makers hoped they would. PC World's list of disappointments Vista makes it in at the top slot, while Leopard comes in at #8. Neither comes as a surprise to me. What happened in 2007 should serve as a cautionary to both Microsoft and Apple. The longer you leave an operating system about before a replacement, the greater the inertia will be when it comes to trying to shift them onto the new version.
What I see as having happened is that in the time that XP and Tiger had been around (remember that while Windows users have been waiting for Vista for a lot longer than Apple fans were waiting for Leopard, the gap between Tiger and Leopard was the largest between Mac OS X launches) users have become too stuck in their ways and have forgotten that being an early adopter generally means that you're the first to stand on the rakes and take one for the team so that those following you in a few months don't have to. I've seen and felt that effect personally. I'd totally forgotten about all the XP RTM problems I'd had all those years ago, so when it came to rolling out Vista, I was comparing an RTM release to an SP2 release. Not only that, but there's years worth of driver maturity and workarounds to take into account. This isn't an excuse, it's just fact.
Microsoft's had it particularly hard with Vista because after years of development and months of hype, people had been led to believe that they were in for something special. What they weren't ready for the huge paradigm shift that Vista bought with it (and when it comes to Office 2007 you get a double-helping of paradigm shift), not just in the way we work, but in what would work. Apple, by comparison, had it easier because Leopard represented another step in the evolution of Mac OS X rather than a sweeping change.