Here's a quote from a Forrester article that I think best sums up the Windows Server 2012 story:   "It appears to me, based on numerous contacts with Microsoft and conversations with several Beta users, that this is essentially a complete rewrite of the OS, involving 1000s of developers over at least five years and probably somewhere between 40 and 50 million lines of code."  All that I would add to this is, "And they got it right." :)  Read the entire article here: http://blogs.forrester.com/richard_fichera/12-09-05-microsoft_announces_windows_server_2012