If I remember correctly, there was much hype before Vista was released two, but it in real world usage, it soon slowed down a perfectly good computer within 1-2 months. I hope this is not the case with Windows 7 too.
My only real gripe is that it's expensive and I've gotten used to running the super mega ultimate edition or whatever it's called. It's just an OS, sell it for $99 and have done with it instead of pointless market segmentation which drives people to the torrents. You'll note that these criticisms are all to do with corporate rather than technical.
So it should (depending on OEM crapware, etc) be a bit snappier.