First off, this is a great game. Great story, great soundtrack, and great graphics (for when it was made). Besides that, this game really should not be rated M. It does have a small amount of swearing, but really nothing a sixth grader can't handle, and I've heard much worse in games rated T. The violence really isn't much to be worried about, either. The player shoots at aliens that have humorous dialogue and are almost cartoons. When hit, their blood is a bright blue, and is hardly visible. Granted, the player can shoot dead bodies to make more blood come out, but it's still not all that bad. When the player is killed, it simply shows Master Chief lying on a bit of bloodstained ground for a couple seconds before he respawns, again nothing a 13 year old can't handle.
As for the "realistic weapons" that the main review makes them out to be, they really aren't. The more realistic ones include a machine gun, pistol, and sniper rifle. However, the majority of weapons fire plasma or look futuristic like a star wars toy, and the sword is made of blue energy, something you probably won't see in the real world.
All in all, this game does have some swearing and violence, but once your kid is out of elementary school, it should be okay. Unless your child is likely to take their dad's plasma gun to school and start looking for aliens to kill, you won't have a problem.