Before even mentioning the new features of VirtualBox 4.0 Beta 1, Oracle's Frank Mehnert mentioned that some functionality is being moved out to "external packages" rather than as part of the core VirtualBox virtualization software. Right now there's just one external package for VirtualBox 4.0 and it's the PUEL extpack, which is where the USB 2.0, RDP Server, and PXE bootloader support reside.
Nothing else was said about the objective of these VirtualBox 4.0 external packages, but it may be a bit troubling for those looking at VirtualBox as a free and open-source virtualization stack. Oracle could potentially begin selling these "external packages" to users that provide core but not necessarily required functionality for virtualizing guest operating systems. Support for USB 2.0 in VirtualBox guest machines is pretty core functionality for a desktop user that is somewhat surprising they would move out into one of these external packages.
Besides selling external packages for VirtualBox, Oracle could also use this "feature" as a way to provide more features but only via closed-source external packages. This though is just my hypothesis, but based upon how Oracle has handled other "open" projects they received via the Sun Microsystems acquisition, such a move would certainly be possible. We'll just have to wait and see though what happens.
Anyhow, in terms of new features for VirtualBox 4.0, improvements include new settings/disk file-layout for VM portability, OVA (Open Virtualization Format Archive) support, Java language bindings, support for the ICH9 chipset as a virtualized option, Intel HD audio is available as virtualized guest audio, there's a redesigned user interface with guest window preview, a new display mode as a down-scaled guest display, support for limiting a virtual machine's CPU time and I/O bandwidth, support for a-synchronous I/O on iSCSI/VMDK/VHD/Parallels images, and support for resizing VDI/VHD images. These are just the most interesting features, but there's a few other enhancements too in this first beta.
Oracle VM VirtualBox 4.0.0 Beta 1 also has various bug-fixes, including GUI tweaks, 3D support fixes, 2D video acceleration support in multi-monitor layouts, and various Linux and guest additions.
The release announcement and download links for VirtualBox 4.0 Beta 1 can be found in the VirtualBox Forums.