Storage I/O Control allows the prioritization of storage disk access using shares across hosts in a cluster - kicks in when congestion occurs (exceeded latency) and dynamically allocates portions of hosts I/O queues to VMs running on the vSphere hosts based on shares assigned to the VMs.
Prior to 5.1 the default latency threshold for storage I/O control (SIOC) is 30msecs. With 5.1 rather than using a default/user selection for latency threshold, SIOC now can automatically determine the best threshold for a datastore.
SIOC is in addition to the basic disk shares and limit functionality which allows you to manage basic disk access priority for vm on the same host (not across hosts - so a single host with lower priority vm could congest I/O paths).
vSphere 5.0 extended Storage I/O Control to also support NFS datastores.