If you are experiencing crashes or hangs in Unity 4.2 or 4.3 running on Windows 8.1, particularly when importing assets or transferring over an older project, take a look at this:
We've hit this issue on a couple of systems. Usually, we can get around the problem by setting the "-force-d3d11" tag on the Unity icon start arguments. Which seems to work on the desktop icon, but not if you pin to the taskbar, which is annoying.
It is also a good idea to make sure DirectX and graphics drivers are up-to-date, which seemed to help on one of our systems.