The lights are on
I doubt it is language. Mainly I think it is the severity of the violence and the amount of damage/blood/gore that is produced. I mean, yeah, Mortal Combat has you smashing skulls and ripping out spines, and Halo is just shooting aliens and other spartians with a little blood, but they both manage to get M ratings. It's up to the discrecion of the ESRB, but some developers should avoid using too much violence and gore, but I guess whatever sells, and tells the story.