On September 4th, 2012 Windows Server 2012 went RTM and is available for download in MSDN and Technet. To get up to speed there are some great resources available on the product and the huge amount of changes and enhancements Microsoft has put into this release.
To get a handle on it all Microsoft Press has made the Introducing Windows Server 2012 RTM edition available for free. You can download it as a PDF, Mobi or ePub format so there is no excuse not to go over to the site right now and get your free copy!
In terms of features, the Hyper-V update, PowerShell 3.0, Storage and Network updates are really important along with the obvious "cloud" enablement that Microsoft has been touting for awhile.
On a separate note, I haven't had time to install Windows 8 RTM onto my primary work laptop and I am considering waiting until the new Window 8 Intel specific hardware comes out after the new year which is about when my refresh cycle is up anyway. My other option is picking up a cost effective SSD and doing a clean build. In the meantime if I really need to test things out I have my Windows To Go stick to play with Windows 8 on my laptop with no fear of damaging my system or losing any data.