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

Support for cloning of VMs (bug #5853, see the manual for more information): full clones can be created through the GUI and VBoxManage, linked clones only through VBoxManage
GUI: enhanced wizard for creating new virtual disks
GUI: new wizard for copying virtual disks
GUI: keep the aspect ratio in scale mode (Windows and OSX hosts only; bug #7822)
VMM: raised the memory limit for 64-bit hosts to 1TB
Experimental support for PCI passthrough for Linux hosts, see the manual for more information
Windows guests: Experimental WDDM graphics driver, supporting Windows Aero (bug #4607) and providing Direct3D support using a cleaner approach (no need to install the guest drivers in Safe Mode anymore)
Guest Additions: status of modules and features can now be queried separately by the frontends
Networking: new network attachment mode "Generic Driver", which offers an open plugin architecture for arbitrary and separately distributable virtual network implementations
Host-only Networking: fixed host crash in kernels prior to 2.6.29
New Networking Mode UDP Tunnel: allows to interconnect VMs running on different hosts easily and transparently, see the manual for more information
Experimental support for SATA hard disk hotplugging available with VBoxManage
Solaris hosts: New Crossbow based bridged networking driver for Solaris 11 build 159 and above
In addition, the following items were fixed and/or added:

VMM: more SMP timer fixes
VMM: fixed sporadic recompiler crashes with SMP guests
VMM: many small fixes
GUI: when reverting to a snapshot, ask for taking a snapshot of the current state
GUI: added a View menu
GUI: added a setting for the promiscuous mode policy for internal networks, bridged networks and host-only networks
GUI: added slider for setting the CPU execution cap allowing to limit the amount of CPU time spent for the execution of the guest, see the manual for more information.
GUI: the VM description is editable during the runtime of a VM (bug #1551)
GUI: added proxy settings (bug #2870)
GUI: made the number of SATA ports configurable
GUI: decrease time before showing the VM configuration dialog
VBoxManage: more convenient configuration of storage controller attachments by automatically determining the port or device parameter when a storage controller has only one port or device per port
VBoxManage: changed syntax of the guestcontrol command group, fixed various bugs, removed obsolete options
VBoxBalloonCtrl: new service for automatic dynamic adjustment of the balloon size for running VMs
Settings: machine names and snapshot names are not allowed to be a valid UUID
Settings: provide better diagnostics if a single medium is used twice in a VM configuration
Settings: provide better diagnostics for errors in medium create/merge/clone operations, and fix memory leaks in error cases
Storage: ATA/SATA drives can be marked as non-rotational, i.e. the guest OS will detect them as a SSD if supported, which can improve performance
Storage: virtual CD/DVD images will be detached if the guest ejects the medium, unless the drive is marked to handle ejects only on a temporary basis
Storage: the medium UUID can be changed again when attaching a medium for the first time, which allows using images which are exact duplicates including the UUID
Storage: fixed possible data corruption under certain circumstances with VHD and Parallels images (bug #9150)
Storage: fixed unnecessary expansion when cloning differential images in VDI format
Storage: fixed detection code to handle empty files for VDI and VMDK format
Storage: fixed access to CD/DVD images beyond 4GB when using the SATA controller (bug #8592)
Floppy: several bugs have been fixed
Floppy: make it possible to unmount a host floppy disk (bug #6651)
BIOS: disk-related structures are now checksummed correctly (bug #8739)
USB: many fixes for the Windows USB host driver
NAT: reduced memory footprint
Networking: fixed the problem with segmentation offloading when several VMs are transmitting in parallel
Networking: workaround for a bug in wireshark when operating directly on a capture file created by VirtualBox
Serial: announce the serial devices in the ACPI tables to make Windows guests find the virtual hardware (bug #7411)
VRDP: support for TLS connections (see the manual for more information)
VRDP: support for multimonitor client configurations with MS RDP clients
VRDP: fixed a rare screen corruption
3D support: fixed GL_VERSION string for different locales (bug #8916)
Webservice: fixed timeout handling with HTTP 1.1 keepalive, and be more robust when connections fail
VBoxSVC: fixed regression when several clients trigger autostart simultaneously
Main: fixed incorrect handling of the medium location for media which are not file based (e.g. iSCSI), which resulted in confusing location values in many places
JAX-WS client bindings: fixed resource leak
Sources: fixed USB 2.0 support using extension packs for non-official builds
Mac OS X hosts: fixed non-VT-x mode on Lion hosts
Windows hosts: fixed copy'n'paste in the GUI and for the VM window (bug #4491)
Windows hosts (64-bit only): enabled removing of all mediums when removing a VM
Windows hosts (64-bit only): enabled live snapshot deletion
Windows hosts: use native controls in the installer (bug #5520)
Solaris hosts: fixed preemption issue with Solaris 11 hosts (builds 166 and above)
Solaris hosts: better control of USB device access on Solaris 11 hosts
Guest Additions: improved driver installation on Windows guests
Guest Additions: fixed high CPU usage while executing guest programs from the host
Solaris Additions: fixed automounting of shared folders (bug #8014)