It probably says something about my own wasted youth that I can be watching a trailer in 2013 and my first thought is: "Wait, wasn't this part of the premise for the shitty 'Double Dragon' movie?" (I checked, it was.)
Anyway, the premise behind "The Purge" is that at some point in the near future the United States has turned itself into a well-behaved utopia by instituting an annual one night "get out of jail free" event wherein all police and/or emergency services are suspended and everyone so-inclined gets a free pass to rob, murder, rape, destroy or whatever anyone and anything they want without fear of legal reprisal. In the film, a suburban family takes pity on a guy stuck outside during this year's Purge, inviting the wrath of the Purge-participants who were hunting him.
Great pitch, to be certain - but to be honest I lost a lot of interest once the Slasher Movie crew turned up. The real horror (and real promise) of the setup should be the revelation of what absolute bastards the "nice normal people" of the world turn into when you take away the chance they'll get punished; so having the heavies for this turn out to be a pack of Arkham Asylum goons who'd likely be out pulling versions of this shit in a world without a Purge feels a lot like chickening-out of something more subversive... though to be fair it's extremely likely that the point of the "Strangers"-knockoff masks is so the film can pull "shocking" reveals ("OMG! It's the mailman/grocery-clerk/barber!") later.