Today I found a strange difference between the Task Manager in Windows Server 2012 and Windows 8.

In Windows Server 2012 you only see the tabs CPU and Memory by default:

Windows Server 2012

In Windows 8 we see CPU, Memory, Disk and Network

Windows 8

I asked this question on the Technet Forums, and got after a few weeks got a final answer from Syed Yusuf who is working for Microsoft R&D
This is because disk metrics are disabled by default on Windows Server 2012 because of significant performance impact, however, it is enabled in Windows 8.Use Resource Monitor to gauge Disk and Network IO usage.
Technet Forum topic with the answer: Task Manager difference between Windows 8 and Windows Server 2012