A: This is an interesting question. Windows Server 2012 Standard and Windows Server 2012 Datacenter are basically the same OS with the same roles, features, and capabilities. The only difference is virtual instance rights: Standard has two, while Datacenter has an unlimited number.
This means Standard now has the failover cluster feature. Technically, you can have a cluster with a mix of Standard and Datacenter in a single Windows Server 2012 failover cluster, which the screen shot of my cluster validation of two nodes (one Standard and one Datacenter) shows.
Failover Cluster with Windows Server 2012 Standard and Datacenter
The problem is licensing. Windows Server 2012 Standard provides only two virtual instance rights while Datacenter gives unlimited. The point of a cluster is to be able to freely move resources between the hosts in the cluster.
The host running Standard has licenses only for two virtual machines (VMs). Those two VMs could be moved to a server running Datacenter (since the Datacenter box has unlimited VM instance rights), then moved back to the Standard server. Virtual instance rights are specific to the physical host and can't be freely moved around.
This means that while technically you can mix Standard and Datacenter editions in a single cluster, from a licensing perspective and for maximum flexibility it's not a good idea.