So, the differences in my case are:
- I can connect to the guest via the host or from the guest to the host over IPv4.
- I can ping the host from the guest or vice-versa via IPv4 and IPv6.
- I cannot connect to any service on the guest from the host or from the guest to the host regardless of protocol.
- The problem disappears when I revert to VirtualBox 5.0.2. But I encounter the race condition that occurs during launch where it takes 4-5 times to launch a guest before it'll start without error under Windows 10.
If you're planning on using VirtualBox any time soon under Windows 10, be aware that it should work unless you're using it to test IPv6 or any services that utilize IPv6.
This isn't resolved as of VirtualBox 5.0.5-r102614 either.