Iv said it before and ill say it again, Vista to me felt slightly unfinished, like there was alot of stuff that was still in "beta" mode (start menu search for instance) Its almost like Ballmer just went "s**t its been 5 years, just shove out what we've got, and call it... Vista, yeah Vista"
Vista was percived as a failure, not because of Microsoft, but because of the software designers and hardware manufacturors,
eveyone "in the know" had code for Vista a whole YEAR before release, but nobody bothered to release drivers
, combine that with the slight unstableness you tend to get with a brand new Windows OS anyway (7 excluded from this) and THAT is why Vista bombed, Vista itself wasn't bad, once you got used to it, the thing that p***ed me off, was those PC companies selling PC's with 1GB of Ram and Vista, sure Vista will run on 1GB of Ram, just, but dont expect to get anything productive done.
If Microsoft made one mistake with Vista, it was making it incompatible with XP drivers and software, they must have known that alot of companies just wouldn't bother updating, it should have been backwards compatible.
Lastly, I am truly thankful for Vista because it led to its little big (little in terms of resources used compared to Vista, big in terms of stuff in it) brother Windows 7