Nano Server will be designed to cause fewer disruptions, including fewer patches, faster restarts, better security, and improved resource utilization, to ensure it is highly optimized for the Cloud and DevOps. Additionally, Nano Server will be componentized, allowing you to only install the pieces you want to run without a multitude of additional modules not necessary to operations.
According to Microsoft, even in the initial builds of Nano Server, it offers:
93 percent lower VHD size
92 percent fewer critical bulletins
80 percent fewer reboots
Microsoft has accomplished this by completely removing the GUI stack, 32 bit support, MSI, and default Server Core pieces. To operate Nano Server, there is no local login, only management remotely through WMI and PowerShell.
Read the full description and watch a demo: Microsoft Announces Nano Server for Modern Apps and Cloud
I'll be at both BUILD and Ignite this year and will make sure this particular topic is high on my list of coverage. Exciting times.