Microsoft has made available for download the release notes for Windows 8.1 and Windows Server 2012. Each document is about two pages long and contains some pieces you'll want to read through before installing either OS to alleviate issues.
The issues stated in the docs are not huge, but it's well worth your time to read through them before installing the server or client OS. The client doc contains information around some IE11 limitations, but the server doc has some additional information about installing on a VM successfully.
A funny note: the Microsoft download site makes the issues contained in the docs sound horribly critical and almost scary. From the download page:
Critical issues that might require avoidance or workaround to get the operating system installed and running.
You can download both docs here: Release Notes: Important Issues in Windows Server 2012 R2; Release Notes: Important Issues in Windows 8.1