Vista flopped - and mainly because it was shite (cue a couple of people rising to defend it).
Let's face it, It wasn't radically different from XP from a user interface point of view (start menu, desktop, all "check"). Furthermore, there had been a wait of over 5 years since XP was released (longest in the history of Windows). The world was ready for a new version of Windows, just not for the sluggish, buggy and annoying turd that was Vista. And let's face it, it was Vista's crap reputation which sank it, not because everyone suddenly started praising XP as being the best thing since sliced bread!
For Windows 8, it's a different story. Windows 7 was only released 3 years ago and for some people, they've only just finished upgrading to it (especially for businesses). The need for a new version of Windows is less, and this is likely to affect adoption rates - regardless of how good or bad Windows 8 is. We've no plans to move to Windows 8 where I work, but that's mainly because I'm still busy removing the last straggling XP machines.
EDITED: 21 Nov 2012 21:33 by DAVE!!