Just noticed that WFA 2.1 install guide states the EE Enterprise Edition is "recommended". We're installing it on a Standard Edition and haven't seen problems so far but wondering why EE is recommended.
To add to Sean's response, yes, typically what's officially recommended is based on what has been tested and confirmed internally. I'm honestly not aware of the various platforms leveraged for the QA & Validation processes, but I can tell you I've run WFA on a variety of flavors of Windows over the years. Windows XP*, Windows 7* 32bit & 64bit, Windows Server 2003*, Windows Server 2008*. I'm now using Windows Server 2012* VMs, and WFA is working just fine.
I WILL say that I DO hope that the QA folks start standing up some Windows Server 2012 VMs soon, so they can recommend both a Windows 2008 AND a Windows 2012 configuration for future versions of WFA. Many of our customers have already moved or will soon be moving to a flavor of Windows Sever 2012.