This version is a major update. The following major new features were added:

Reorganization of VirtualBox into a base package and Extension Packs; see chapter 1.5, Installing VirtualBox and extension packs, see the manual for more information
New settings/disk file layout for VM portability; see chapter 10.1, Where VirtualBox stores its files, see the manual for more information
Major rework of the GUI (now called “VirtualBox Manager”):
Redesigned user interface with guest window preview (also for screenshots)
New “scale” display mode with scaled guest display; see chapter 1.8.5, Resizing the machine’s window, see the manual for more information
Support for creating and starting .vbox desktop shortcuts (bug #1889)
The VM list is now sortable
Machines can now be deleted easily without a trace including snapshots and saved states, and optionally including attached disk images (bug #5511; also, VBoxManage unregistervm --delete can do the same now)
Built-in creation of desktop file shortcuts to start VMs on double click (bug #2322)
VMM: support more than 1.5/2 GB guest RAM on 32-bit hosts
New virtual hardware:
Intel ICH9 chipset with three PCI buses, PCI Express and Message Signaled Interrupts (MSI); see chapter 3.4.1, “Motherboard” tab, see the manual for more information
Intel HD Audio, for better support of modern guest operating systems (e.g. 64-bit Windows; bug #2785)
Improvements to OVF support (see chapter 1.12, Importing and exporting virtual machines, see the manual for more information):
Open Virtualization Format Archive (OVA) support
Significant performance improvements during export and import
Creation of the manifest file on export is optional now
Imported disks can have formats other than VMDK
Resource control: added support for limiting a VM’s CPU time and IO bandwidth; see chapter 5.8, Limiting bandwidth for disk images, see the manual for more information
Storage: support asynchronous I/O for iSCSI, VMDK, VHD and Parallels images
Storage: support for resizing VDI and VHD images; see chapter 8.21, VBoxManage modifyhd, see the manual for more information.
Guest Additions: support for multiple virtual screens in Linux and Solaris guests using X.Org server 1.3 and later
Language bindings: uniform Java bindings for both local (COM/XPCOM) and remote (SOAP) invocation APIs