Alt+Tab is not switching between windows on Ubuntu 12.04 (Precise Pangolin)

Well, just to finalize the answer, a reboot has done the trick for me after doing the following:

I went to system-settings -> keyboard -> shortcuts and almost all of them were disabled. I assigned "Alt+Tab" to "Switch applications". Then even though the problem specified in one of the comments above persisted, a reboot has solved the problem and I'm able to switch between windows without any problem.

Edit: Sometimes you have to reboot multiple times.

I use Super + W and select from current open programs. not the best solution to this issue but it helps.

I'm also using Gnome Classic (with effects or without them). I have reported the bug about Alt+Tab, but Sebastien Bacher said that it is not a bug. I found that it's duplicate of 971051. Let's make these bugs heater!

As temporary solution we can install CompizConfig Settings Manager and select 'Application Switcher' or 'Static Application Switcher' from Window Management group.