There are a number of key reasons NOT to use Windows Server 2008 R2 for AlwaysOn Availability Groups (AGs), including:
Windows Server 2012 R2 DataCenter Edition picks up a roughly 28 percent price increase, but Standard Edition stays roughly the same price. The only caveat, of course, is that Windows Server 2012 and above, need to be licensed via 2-Processor Packs—which is a pain for smaller servers—and that CALs are no longer included in most pricing options as part of the license for Windows Server 2012 and above. Or, in short, licensing Windows is always complex, convoluted, and cryptic enough that you typically need to spend a solid 20 minutes perusing documentation before you can actually feel that you get a semblance of a grip on what you need to buy/license to provision new servers.
The point I’m trying to get across, though, is that in most environments, Windows Server 2012/2012 R2 Standard Edition is enough to keep SQL Server AlwaysOn Availability Groups adequately and amply powered and happy. In fact, the only differentiator between the DataCenter Edition and the Standard Edition is that DataCenter Edition provides unlimited hosting of virtual machines, or VMs (i.e., license the physical processors and run as many VMs as your hardware can handle) whereas, Standard Edition allows only 2 VMs per License 2-Pack (with any further VMs requiring their own, actual, licenses), and is really geared more towards traditional or physical workloads—which is what you're going to want in most cases for your AGs, anyhow.
Other than that, BOTH versions of Windows Server support the same amount of hardware, throughput, and are full-featured versions of Windows Server (i.e., they’re not artificially restricted in any way).
The difference in pricing, though, is pretty substantial. Not including CALS, Windows Server 2012 R2 Standard Edition typically runs around $882 for 2 processors, while Windows Server 2012 R2 DataCenter Edition typically costs around $6,100 for 2 processors (Windows Server 2012 DataCenter Edition cost roughly 28 percent cheaper at around $4,800 for 2 processors).
So, in short, Windows Server 2012 or Windows Server 2012 R2 Standard Edition is a great way to save thousands on licensing—and save yourself some serious headaches when deploying AlwaysOn Availabiliity Groups.