Alright, here's a broad question: What makes Mass Effect an M-rated game to you, and what would be lost if they took the rating down a notch?
Great post, and a great start.
For me it just comes back to this four letter word; tone.
These games have a very adult tone. They deal with adult fear, adult hate, adult love.
And I think if you take away a lot of the elements that help make the world that way; the sexual content and themes, the language, the alcohol, then at best what you can do is turn out a fairly well written story that just does not feel true to life because it doesn't have a voice to it. It becomes something that has to be constructed around the limitations of it's rating and doesn't feel real. It doesn't feel true to life.
In the other thread, when we're talking about swearing, the claim is made that you can use non-swear words to convey the same emotions as swear words, just making them unnecessary. To an extent that is true in that it is physically possible, but it also isn't very true to life. You can write a character who can get all their points across with words that are going to send your audience to thesaurus.com as well as they could with a tirade of expletives, but the tirade of expletives is going to have something the more verbose character wouldn't; they're going to feel authentic. It could give you something like Michael Pena's character in Ant-Man, which works for a comedy but doesn't work with a more serious tone.
That's just one example. The point is, these games need to be rated M to maintain a mature, adult tone. That's where the believably of the series comes from. It isn't in the ridiculous technology fueled by unobtanium. It's in the authenticity of the characters, and that comes from things that they couldn't get away with on a T rating.