Below is a overview of features in vSphere 5.0 and 5.1
Essential: No change. Only the Hypervisor.
Essential Plus: Added vShield Endpoint, vSphere Replication, Hot Add.
Standard: Added those in Essential Plus and vShield Zones, Fault Tolerance, Storage vMotion.
Enterprise & Enterprise Plus: Almost similar.
For Essential and Essential Plus, it still much more applicable to SMB or Remote offices whereby there is no need of the performance guarantee since typically such environment do not have much workloads. The limitation still applies here where Essential kits are restricted to 6 processors or 3 hosts whichever is maximize first and comes with vCenter Essential where features like Linked mode is not present.
With the added features to Standard Edition, does that means Enterprise would not be that significant? Not entirely true.
With Standard edition and above you can match two types of vCenter namely foundation and standard.
vCenter foundation is very applicable to small and remote offices where the requirements does not exceed 3 hosts while vCenter standard does not have such limitation. Also to note Linked mode is only available in vCenter Standard.
vSphere Standard edition would still be more suitable for environment with low workload though Storage vMotion and Fault Tolerance are added. If you look closer, Standard edition does not contain Storage API for Array Integration (VAAI) and similarly it still does not have DRS/DPM.
Without Storage API, the host's resources will be used for storage activities for copy offload, write same offload and hardware assisted locking. Without these, host's resources will be utilized to perform e.g. cloning where file level copying creating load on the host, a data store will be locked when a write by a host to the data store which means delay of storage tasks in such results in performance degrading. Storage vMotion in short in Standard Edition is more applicable in scenario for putting a data store for maintenance use rather than towards daily operations. It is also provided to compliment with "share nothing" vMotion function.
Without DRS, you would need to do manual auto load balancing. With high number of workloads, not does this increase operations overheads but also decrease resource utilization as manual calculation would need to be done to find how workloads can be balanced.
Also there is no Storage Multipathing Policy (MPP) support for Standard. With that, if this is understand correctly you will only be entitled to Native Multipathing (NMP) in vSphere. In such, you will not be able to use vendor specific MPP e.g. EMC PowerPath as the vStorage APIs for MPP is not included for this edition.
If you decide to move to standard, your consolidation ratio may not be as high as what is possible on Enterprise edition and above. With that in mind, do cater into your design on the additional physical resource requirements. So spending less on licenses might results in spending more on hardware which the end result might just be more expensive.
So which editions would be suitable for you. You can try out the VMware vSphere Purchase Advisor here.
With the additional features to Standard edition, it was more to enable the use of the required abilities irregardless of small or big environment. However if the environment is big, then Enterprise and Enterprise Plus should be considered to maximize your resources and lower the operations overheads.
To find out more of vSphere 5.1 and their features descriptions, you can find here. The descriptions are available on clicking the features however the editions recommended are still referring to vSphere 5.0 at the time of writing.