As a 23 year old male, I'm an active gamer that plans to have kids in the future. I've completed Halo 4 by myself on the hardest difficulty (Legendary) and played online a bit. The game has a fantastic, surprising story, thanks to the new developer, 343 Industries. The game is linear and story based, so it's more like a movie versus GTA, where players can potentially choose to deviate from the story and go into strip clubs.
Halo 4, in the best case scenario of positive parenting, can encourage hand-eye coordination, motor skill development, team work, communication skills and perseverance/anger management (on harder difficulties).
If kids play online, they should probably be 15+ if they use voice chat. Everyone else should turn voice off, which I think is possible in the settings. There is blatant profanity, racism, sexism and homophobic remarks from anonymous players online, or at least the potential for such things. Kids should be trained to expect such behavior and ignore these people and leave the particular match if they are really rude. Also, it's possible to mute specific players mid-game and send reports on their behavior.
Cortana appears to be nude, but I'd compare her to Mystique from X-Men. It's a blue, skin-tight suit that doesn't show any anatomical details. But the female form is there for sure. In certain parts, she comes up in 3d hologram form and the player can rotate the image of her. I'd image this might cause giggles in young kids, but not much more.
The game can desensitize players to becoming skilled at getting head shots, or instant kills. These shooting games, if analyzed critically, always make me ask why are these soldiers are going to war and why we're killing these hundreds of troops. It's important to talk about death, what it means and why we might have to kill, in the case of war and protecting our families. Like what happens when we die? It might be a hard conversation to have, but no one thinks twice about how the bodies are piling up and what it ultimately means. Thinking can make you question why you are killing all these people/creatures in games.
Some enemies (The Covenant) in the game are technologically advanced, but religiously insane in their black and white views of the universe. They always seek to gain entrance to heaven by killing everyone in the universe through use of various weapons. It's never a good thing to be so black and white, but perhaps an analysis of fundamentalist religion can be made here.
The forerunner element of the game highlights mankind's flaws, like our agression and manifest destiny for conquest of other countries and the universe itself. Forerunners are the original protectors of the universe and they point out how mankind might lead to the downfall of everything if they're left to their own devices. Parents can draw parallels to the Cuban Missile Crisis and possible self-extinction events. Are we as a species capable of surviving in the future or do we need guidance from a higher authority?
Depending on your view of the military, the Master Chief can be a positive role model only in the sense that he is a tireless military protector of mankind, or of his country/nation (in this case, Earth).
I think important themes of losing your humanity at a cost are prevalent - in the way that Chief and Cortana (the player's assistant, an AI woman) question which one of them is really the machine. In the same way that a military soldier must dehumanize himself and his enemy in order to kill, so too must the Chief look forwards in fairly simplistic ways. Chief (John 117) was engineered in a special science program to be stronger than other humans, but the cost being that he is antisocial and can't relate to other humans. He talks to his AI, Cortana, almost exclusively, to preserve his mind.