Sources: Microsoft , 
quote: Vista was taken from Server 2003 after the Longhorn project failed. It wasn't a really big step. They added some features for content, and wrapped some security functions around it. That was the major change from Sever 2003. Then they went overboard in the UI.
quote: Microsoft has announced that they will begin depreciating the Desktop, and that Metro is the future for Windows. That's certainly a major change for them, as it obsoletes everything from before. You're forced into dealing with Metro no matter how much you want to remain with the Desktop. It is what it is, like it or not.