Mac OS X never shuts down
Does it also happen when you log out? Did you already check system.log?

Is there a logout hook (sudo defaults read com.apple.loginwindow LogoutHook)? I guess not though, because normally when a logout hook does not terminate, there is a solid light gray screen without any process indicator.

You could try unloading the kernel extensions with sudo kextunload -b $bundleid before restarting. Or just move the kext bundles outside /System/Library/Extensions/ and restart once so the kernel extensions won't be loaded.

You could also try terminating different processes before logging out, or for example run for f in {~,}/Library/Launch{Agents,Daemons}/*.plist; do launchctl unload $f; done.