If you plan to deploy Office Web Apps in support of Lync Server 2013 and decide to run in it on Windows Server 2012, you will run into problems if you do not install .NET 3.5
Technet does not call this specific requirement out http://technet.microsoft.com/en-us/library/jj219455.aspx however if you do not install .NET 3.5 you will see the following issue when attempting to upload PowerPoint presentations. “Some Presenting Features are unavailable due to server connectivity issues.”
You will also likely not get Event ID 41034 which say the Web Conferencing Server has successfully discovered the Office Web Apps Server, PowerPoint content is enabled.
On the Office Web App server in question, open a Powershell window and run
"Add-WindowsFeature NET-Framework-Features, NET-Framework-Core"
Patch the server using Windows Update and reboot.
Re-test Sharing Powerpoints and they rendering and you should be able to upload PowerPoint content!
Hopefully Microsoft updates Technet to include this in the pre-requisites soon so you don’t get burned by this.