I don't really know what else besides gore, swearing and sex would make it a mature game. I mean the story about the hive torturing members of the fireteam in the grimoire or people being erased from existance could be pretty disturbing if they wanted it to be. Those stories themselves could be pretty twisted, but I dunno how they would be more so without throwing in blood and guts, people screaming as you watch their torture, going insane as they are erased from time, etc. And that would make it a mature game. I dunno if that would make it a "richer" story though. My guess is most people who voted yes just don't want to have to play with as many younger players, which a mature rating won't really resolve anyways.