1. Massive scale
2. Networking
3. VM Mobility
4. Storage
5. Automation/Manageability
over 2400 PowerShell Commandlets for Windows Server 2012 overall over 150 PowerShell Commandlets just for Hyper-V in Windows Server 2012
6. Performance
7. . . . . .
There are so many more reasons why. Read the Windows Server 2012 Evaluation Guide.