Windows Server - since 2003 - has not been exactly *beastly* in terms of CPU requiremnts for the basic server, and even 2012R2 didn't change that.
The rather shocking thing is that I found using it as a workstation or virtualization server, especially with Hyper-V, generally more stable than either Windows 7 OR Windows 8+ in the same role; however, it turns out that THAT is the case primarily due to what it lacks in comparison.
1. Even with the Desktop Experience (server/workstation role) installed, Windows Server is still not desktop Windows, despite the two having the same base of code since Windows XP Service Pack 2. Noticeably missing from Server 2012R2, compared to Windows 8, is DirectX - which means no desktop gaming on a server or workstation with that OS installed. (Paradoxically, Windows Server is actually a better platform for browser-based gaming, due to increased out-of-the-box security regarding browser extensions, plug-ins, and add-ons.)
2. Virtualization - Like Windows 8+, Server 2012R2 supports Hyper-V; unlike Windows 8+, Extended Processor Table support is an option - not a requirement. That also means that the floor for Windows Server 2012R2 overlaps with Windows 7 (in addition to Windows 8) - for example, you can install Server 2012R2 on as little as a Celeron E3400.
3. Workstation/server I/O - because 2012R2 is a server operating system, it does support features that you would expect FROM a server OS, such as LAN adapter teaming; however, Server 2012R2, like Server 2012, also supports dis-similar-adater teams (such as wired and wireless, or two different wireless adapters). In the case of 2012R2, it also supports most of Windows 8's - if not Windows 7's - drivers (Catalyst Unified from AMD and ForceWare from nVidia are directly supported, for example). Naturally, Server 2012 takes better advantage of hardware RAID than desktop Windows.
4. Lightweight fighter - Server 2012 and 2012R2 are light in terms of how much space the OS itself takes up - in fact, either is lighter than even 7 Home Premium, let alone 7 Ultimate, 8 Pro, or 8 ProWMC, even with Desktop Experience installed. The only feature you'd be lacking (other than DirectX) is Windows Media Center (yes - Server 2012R2 DOES include the Windows App Store - it is installed alongside Desktop Experience, as is, rather amusingly, touch-screen display support).