I'm using Vbox 4.2 for a while and all goes well.

A notice here is that a Win8 VM can be used or installed without the 3D checkbox enabled.
So the 3D acceleration checkbox is not grayed out or forced checked anymore.

It must be also true that Aero got a bit lighter. Themes still get displayed Without 3D acceleration and 128 mb VRAM or less. (not tried with 32mb or something like this but have no intention on crashing my VM since it works nicely).

This is still a problem (was also in version 4.1):
With 3D acceleration, the toolbox dissapears in fullscreen and you cannot minimize or close the VM window. Shutdown the VM it's the only option because Right Ctrl + F (switch from fullscreen) doesn't work. If you search for the toolbox notice that the mouse changes and you can accidentally click on some host files/launchers on that position on screen without seeing them and this is dangerous because you can delete files or mess with something important.

Fixes (many possibilities, each with its own advantages/disadvantages):
-Disable the fullscreen toolbar/toolbox and switch with Right Ctrl + F
-Disable 3D acceleration
It's possible that the problems with missing toolbox in fullscreen or seamless mode gets fixed without 3D acceleration since that it's just a WDDM 3D VBox glitch.
-Uninstall VBox WDDM by rolling back driver (in Device Manager) to MS Basic Diaplay Adapter + This way you can use custom resolutions.

To end this properly I'm just gonna say that the VBox WDDM got better in version 4.2 but you can still keep the basic display with your custom resolutions if that works better.