It just stopped working. After I boot Ubuntu it does not work. When I run
unity --replace it works again, but after restart problem occurs again. And that happens on every reboot. I have some startup apps, but I don't think that can be the problem.
Looks like I had some problems with firefox causing Alt+Tab to stop working. After reinstalling firefox and computer restart it works.
Well, this Alt + Tab window switcher preview is not solved. It stil happens sometimes after the boot. But I just figured that it works, but it is hidden by other windows due to wrong z-index positioning.
There is a bug https://bugs.launchpad.net/ubuntu/+source/unity/+bug/805087