What's New in vSphere 5.5 - Quick Check

Availability

vSphere Data Protection Improvements - Backups can now be restored to an ESXi Host directly without a vCenter. It allows a more granular storage, backup and restore configuration and you can set specific start times for backup jobs.
vSphere App HA - Restarts application services inside a virtual machine

PowerCLI Improvements - Additional Cmdlets for vSphere Tags, Distributed Switches and VM Console. Support for Windows 8/Windows Server 2012
Big Data Extensions - Formerly known as Serengeti. Fully automatically deployment and management of Hadoop Clusters. Supports Apache Hadoop 1.2, Cloudera (CDH3 and CDH4), Greenplum HD, Hortonworks (HDP1.3) and MapR2.1.3.
vCenter Server Appliance Scale - Supports up to 100 hosts or 3000 virtual machines.
vCenter Single Sign-On - Completely rewritten with better redundancy. An external database is no longer required.
vSphere Web Client Improvements - Better performance an some neat features like drag and drop, filters and recent items.

vSAN - vSphere Virtual SAN is an more sophisticated vSphere Storage Appliance. It is more salable, has many more features and runs inside the VMkernel.
vSphere Flash Read Cache - It allows you to use local SSDs in your ESXi Host to offload I/O from the SAN to the local SSD.
PDL AutoRemove - Automatically removes a device from the host that has been detected as permanent lost (Permanent Device Loss).
62TB VMDK - The maximum size of an Virtual Disk has been increased to 62.9TB
VAAI UNMAP Improvements - Now accessible with esxcli and the size is now specified in blocks rather than a percentage.
VMFS Heap Improvements - Solved any concerns when accessing more than 30TB of open files from a single ESXi host.

40Gb NIC Support - Supports the Mellanox ConnectX-3 adapter cards with Virtual Protocol Interconnect (VPI).
Enhanced Host-Level Packet Capture - A Host-based packet capture tool like Wireshark or tcpdump.
Quality of Service Tagging - Support for Differentiated Service Code Point (DCSP) tagging.