Here's how they could have made that exact same movie a really entertaining flick: Make it a psychological thriller, not a goddamn action movie. Think about how brainwashed people would need to be to support something like legalized crime and murder for a night. We need more explanation for how this developed and why these people aren't more like automatons if they're just going to accept this. So either these people are completely brainwashed and unrelatable to the point that we're very disturbed by their thinking OR they're regular people who completely object to The Purge and have still had it imposed on them anyways. Either of these two set-ups would have made this movie more interesting that stereotypical every day people that somehow have no opinions on the concept of The Purge itself.
As for somewhat smaller items:
(a) Fix the fucking dialogue at the beginning of the movie. We don't need Ethan Hawke's character being a complete stereotype. Everything he said was forced and designed to quickly develop his role as the rich guy with a family life limited by his career. Just make him more realistic from go. Make him either a rich guy with a terrible family life or rich guy with a positive family life in spite of his richness. I don't like Sean Penn very much and Sean Penn could have done this believably. Edward Norton could have done it too. I don't even need those two. I either just need your everyday guy reacting to an extraordinary situation or I need a total asshole that is almost impossible to like up until he starts protecting his family.
(b) The daughter shouldn't look like she's 30. This is constantly fucked up in movies and it's distracting. Find an actor that looks their age. We're supposed to believe her relationship with an "older" guy that clearly looks younger than her. You can find a hot girl that looks like she's 18. Additionally...the boyfriend either didn't need to exist at all OR he should have had far greater justification for killing Hawke. Don't throw something that significant in if you're not going to show us why the kid absolutely hates him. The boyfriend should NOT look clean cut, American boy either. Make him young looking and make him look like a loser, asshole. Make him one of the kids from the movie Kids or Bully. Someone that's a fucking loser that the girl is hooked on because he took her virginity or found her in a vulnerable moment and brainwashed her. We need to hate this kid.
(c) The son should NOT have fucking let that homeless guy in. These people have been through this supposedly multiple times. This is such an asinine mistake. I don't care if he's autistic or whatever...that's fine. But he'd have known goddamn well enough not to do this and been trained to shut the fuck up and sit in his room. The kid's autism and sympathy for people or whatever could have led to him making mistakes inside the house...not completely fucking his family against their knowledge. OR make the homeless guy somebody that the family is vaguely familiar with if you're going to insist on getting someone in the house. Give Hawke or the wife some reason to actually want this guy in the house.
(d) The security system is seriously that fucking weak that it can just be pulled off with a truck? Instead of making the security system weak, do what Sandin said...find a creative way to get them out of the house or to make bad decisions. Smoke them out, start a fire, sneak in...whatever. Just don't make the goddamn security system that he built for the entire neighborhood be completely insufficient to stop the very thing it was designed for. Hell..hack the fucking system. Anything other than just tearing the front door off with a regular, standard truck.
(e) Don't turn the movie into a goddamn Bruce Willis action movie with these situations that are set up perfectly for sneak shooting attacks from behind.
(f) Lena Headey fucking sucked in this role. She was AWFUL and she was cold and she wasn't likeable. They needed a warm, caring mother in here that had some surprising level of toughness under fire to her. We needed a Jodie Foster in Silence of the Lambs type of person. Julianne Moore type could have worked here too.
(g) The family needs to have had some demonstrated working knowledge of weaponry. They're too fucking skilled for having zero acknowledged experience with these guns, machetes, etc.