Often it’s the little things…
If you install an evaluation copy of Windows Server 2012 and later try to install the KMS key to make the server permanent, for me at least the activation wizard throws a fit and fails to take the key. Please note this is installing the product key that causes a machine to search out a KMS server, not an actual KMS license (although that’s supposed to work as well).
If you start an elevated command prompt and run DISM /Online /Get-CurrentEdition it should end up saying your current edition. For datacenter the value is ServerDatacenterEval. I think for some reason the wizard doesn’t understand you’re trying to ditch the eval component and rejects the KMS key since it’s technically different.
To fix this, run DISM /Online /Set-Edition:ServerDatacenter /productkey:48HP8-DN98B-MYWDG-T2DCC-8W83P /AcceptEula for datacenter or DISM /Online /Set-Edition:ServerStandard /productkey:XC9B7-NBPP2-83J2H-RHMBY-92BT4 /AcceptEulafor standard from that same elevated command prompt and reboot when prompted.
This will leave the 2012 evaluation install in the same situation it would have been if you’d started from a full copy of the media, and it will attempt KMS activation going forward. You can run slmgr.vbs /ato from an elevated command prompt if you want to kick it off immediately (after the reboots complete).
The list of all the KMS product keys is available here http://technet.microsoft.com/en-us/library/jj612867.aspx
A bit of vEverything 