Comparison and Difference between vSphere 5.1 and 5.5

This is one general question which can be asked to you in any interview but rather more being a VMware Administrator you should understand the new features introduced in the later version of VMware vSphere 5.5 as compared to older version 5.1.
So in order to understand the difference between vSphere 5.5 and 5.1 I believe it will be better if we concentrate on that part of new features added to vSphere 5.5 which will automatically answer the former question.
Let us review the new features added in VMware vSphere 5.5 and also some enhanced features from older versions.

vSphere ESXi Hypervisor Enahncements

Hot Pluggable PCIe SSD Devices
Support for Reliable Memory Technology
Enhancements for CPU C-States

VMware vCenter Server Enhancements

VMware vSphere Web Client
VMware vCenter Server Appliance
vSphere App HA
Virtual Machine–Virtual Machine Affinity Rules Enhancements
vSphere Big Data Extensions

vSphere Storage Enhancements

Support for 62TB VMDK
MSCS Updates
vSphere 5.1 Feature Updates
16GB E2E support
vSphere Flash Read Cache

vSphere Networking Enhancements

Link Aggregation Control Protocol Enhancements
Traffic Filtering
Quality of Service Tagging
40GB NIC support

vSphere ESXi Hypervisor Enahncements

Hot Pluggable PCie SSD Devices
In earlier versions VMware had a feature to swap storage devices such as SATA and SAS hard disks without affecting the running virtual machines reducing the amount of downtime. With vSphere 5.5 users can now also hot-add or hot-remove an Solid State Disks (SSD) while a vSphere host is running.
Support for Reliable Memory Technology
There is a feature in CPU hardware which can provide the details of a portion of the memory which is considered as more "reliable" with respect to to other sections of memory. vSphere 5.5 is utilizing this feature to get memory information and runs the VMkerenl on this section of memory to reduce the slightest probability of any memory crash or failure which in case could futher lead to adverse effects to VMware Hypervisor.
Enhancements for CPU C-States
In vSphere 5.1 and earlier versions CPU P state was used which are responsible for lowering the CPU multiplier and CPU voltage when there is no work load. But with vSphere 5.5 CPU C-State has been introduced which has advanced CPU current lowering technologies. Another potential benefit of
reduced power consumption is with inherent increased performance.

VMware vCenter Server Enhancements

VMware vSphere Web Client
Enhanced features have been added to vSphere web client also providing full support for Mac OS X. The vSphere web client also includes the following new features
Drag and Drop: Administrators now can drag and drop objects from the center panel onto the vSphere inventory, enabling them to quickly perform bulk actions.
Filters: Administrators can now select properties on a list of displayed objects and selected filters to meet specific search criteria. Displayed objects are dynamically updated to reflect the specific filters selected.
Recent Items: Administrators spend most of their day working on a handful of objects. The new recent items navigation aid enables them to navigate with ease, typically by using one click between their most commonly used objects.
vCenter Server Appliance
With the release of vSphere 5.5, the vCenter Server Appliance addresses this with a re engineered, embedded vPostgres database that can now support as many as 100 vSphere hosts or 3,000 virtual machines (with appropriate sizing). With new scalability maximums and simplified vCenter Server deployment and management, the vCenter Server Appliance offers an attractive alternative to the Windows version of vCenter Server when planning a new installation of vCenter Server 5.5.
vSphere App HA
This is an additional feature added to vSphere 5.5. In earlier versions we had vSphere HA which enabled virtual machine monitoring and in case of any heart beat failures within a given amount of time the vShphere HA would reset the vm. But using vSphere App HA any critical application service and be monitored and in case of any failure the application or the virtual machine can be restarted.
Virtual Machine–Virtual Machine Affinity Rules Enhancements
As we know the usage of DRS in earlier version of VMware vSphere but in vSphere 5.5 an additional rule is added to DRS where user can define the virtual machines which should be kept together on same host or separate hosts. This rule is termed as virtual machine–virtual machine anti affinity rule. So now before attempting to move any vm during a resource outage this rule plays its part and migrates the vm accordingly.
VMware vSphere Data Protection Enhancements
New enhanced features has been added to VMware vpshere Data Protection which is a backup and recovery solution for VMware virtual machines. The following enhancements were made

Direct-to-host emergency restore:
Backup and restore of individual virtual machine hard disks (.vmdk files): Individual .vmdk files can be selected for backup and restore operations.
Replication to EMC Avamar
Flexible storage placement
Mounting of existing backup data storage to new appliance
Scheduling granularity

vSphere Big Data Extensions
BDE is a tool that enables administrators to deploy and manage Hadoop clusters on vSphere from a familiar vSphere Web Client interface.
BDE performs the following functions on the virtual Hadoop clusters it manages:

Creates, deletes, starts, stops and resizes clusters
Controls resource usage of Hadoop clusters
Specifies physical server topology information
Manages the Hadoop distributions available to BDE users
Automatically scales

vSphere Storage Enhancements

Support for 62TB VMDK
VMware is increasing the maximum size of a virtual machine disk file (VMDK) in vSphere 5.5. The previous limit was 2TB—512 bytes. The new limit is 62TB. The maximum size of a virtual Raw Device Mapping (RDM) is also increasing, from 2TB—512 bytes to 62TB. Virtual machine snapshots also support this new size for delta disks that are created when a snapshot is taken of the virtual machine.
Enhancements in MSCS
In vSphere 5.5, VMware supports the following features related to Microsoft Cluster Service (MSCS):

Microsoft Windows 2012
Round-robin path policy for shared storage
iSCSI protocol for shared storage
Fibre Channel over Ethernet (FCoE) protocol for shared storage

16GB E2E Support
In vSphere 5.1, VMware introduced support to run these HBAs at 16Gb. However, there is no support for full, end-to-end 16Gb connectivity from host to array. To get full bandwidth, a number of 8Gb connections must be created from the switch to the storage array.
In vSphere 5.5, VMware introduces 16Gb end-to-end FC support. Both the HBAs and array controllers can run at 16Gb as long as the FC switch between the initiator and target supports it.

vSphere Networking Enhancements

Link Aggregation Control Protocol (LACP) Enhancements
LACP is a method to bundle several physical network links to form logical channel for increased bandwidth and redunancy purposes. The fllowing key enhancements are available in vSPhere Distributed Switch with vSphere 5.5:

22 new hashing algorith options are available
64 LAG (Link Aggregation Group) per host

Traffic Filtering
Traffic filtering is the ability to filter packets based on the various parameters of the packet header. This capability is also referred to as access control lists (ACLs), and it is used to provide port-level security.
Quality of Service tagging
QOS is responsible for responsible for differentiating traffic importance and helps reserving bandwidth accordingly. VMware has supported 802.1p tagging on VDS since vSphere 5.1. The 802.1p tag is inserted in the Ethernet header before the packet is sent out on the physical network. In vSphere 5.5, the DSCP marking support enables users to insert tags in the IP header. IP header–level tagging helps in layer 3 environments, where physical routers function better with an IP header tag than with an Ethernet header tag.
40GB NIC Support
Support for 40GB NICs on the vSphere platform enables users to take advantage of higher bandwidth pipesto the servers.

Summary

So this was a brief article on few of the enhancements and new features added in vSphere 5.5 as compared to earlier versions. To summarize this article I have separated the points in two sub sections as shown below
New features in vSphere 5.5

Hot plug SSD PCIe devices
vSphere App HA
Reliable memory Technology:
vSphere DRS virtual machine–virtual machine affinity rule
vSphere Big Data Extensions
Support for 62TB VMDK
16GB E2E support
40GB NIC Support

Enhanced features in vSphere 5.5

Enhancements to CPU C-states
Expanded virtual graphics support
vSphere Web Client platform support and UI improvements
MSCS updates
PDL AutoRemove
Improved LACP capabilities
Traffic filtering
Quality of Service tagging