The vSphere Web Client has seen a number of enhancements in this release. Many reflect user feedback, such as the "10 most recent objects" list shown in the screenshot. Other improvements to the user experience include the new drag-and-drop support and the ability to filter search results for large installations.
The vCenter Server Appliance, meanwhile, gets a scalability boost. Previous versions supported a limited number of hosts and VMs, but these limits have been increased to 500 hosts and up to 5,000 VMs.
VMware has also poured a good deal of effort into making vCenter Single Sign-On simpler to install and easier to scale across multiple vCenter Server instances. Version 5.5 will even include a suite of diagnostic tools.