Agree with a lot of what's been said above. Given 12 hours, most "regular" folks wouldn't have the time to work up the courage to do something major (murder, rape, etc.), but you'd probably see a bunch of smaller stuff (theft, running red lights, downloading pirated movies). What would be interesting is if it were a regular thing- such as 12 hours every month. Then people could really plan/scheme. Or little things that happen each month would have different consequences. Your boss pass over you for a promotion at work, normally you suck it up and get over it. But if you know that next Friday you could take a baseball bat to his/her car without consequence, you'd probably do it.
But I agree, what prevents people from murder and violent crime isn't really laws or the threat of punishment, it's an innate/divine sense of justice. Though you'd probably end up with a bunch of "Robin Hoods" who would do illegal things in the name of justice (letting people out of jail, stealing from the rich and giving to those in need).