While I haven't played with too many of the new features, I did stumble across an issue with VirtualBox that is a bit of a nuisance. The gist of it is in this ticket, but in general Mavericks fully stops using the legacy StartupItems piece of OS X.
This is important for VirtualBox as, among other things, this is how it starts its networking services that provides the ability for host-only adapters. A bit of a nuisance and hopefully the issue gets resolved soon.StartupItems no longer exist in Mac OS 10.9 Mavericks. StartupItems was deprecated 8 years ago. Host-only adapters (and possibly other functionality) cannot be used in Mac OS 10.9.Temporary work around: sudo /Library/StartupItems/VirtualBox/VirtualBox start (Must be run after each restart)