To me there are 2 features in vSphere 5 that make it a must have! The first is ability for failback in SRM 5. Anyone who uses SRM knows the pain associated with failback. In addition to the failback feature, SRM 5 introduces a single UI for all SRM instances!
The second, and my absolute favorite is Storage DRS!!! Our primary production VMware environment consists of ~65TB of storage. In the past 18 months, we have migrated that storage 3 times. The first two times were between SANs environments. The third migration was from 500GB datastores to 1TB datastores, a project that is still in progress. With vSphere 4.x we have had to use PowerCLI to automate those migrations (see this VMware Communities Site for an example script). This has worked well, but requires oversight due to provisioned capacity, free space, affifinty, and VMs per datastore. Storage DRS in vSphere 5 eliminates the need for manual storage migrations and PowerCLI scripts. In addition, it adds a great deal of important functionality such as I/O load balancing and Datastore Clustering.