Great column as always. Thought provoking and such. Anyway, I've long had my own definition of what is "mature," and I believe it is best illustrated through comparison.
I say that themes like sex, drugs, violence, and so forth are not inherently "mature," but rather merely "adult," as in, not for the kids. Duke Nukem has all three of those, and it's intentionally immature. It is merely "adult."
In order to be "mature," as I see it, something must be made to provoke thought or genuine emotion in someone. That pencil scene was quite mature, but you can have "mature" themes without them necessarily being adult themes. My favorite example would be the Klonoa games. It's a cutesy, colorful platformer that's good for all ages yet it deals with very serious themes such as friendship, betrayal, and death (outright murder, actually). The way it treats these themes is emotionally affecting and thought provoking, and thus I call them mature. They're appropriate for people of all ages, and they are very mature.
So! I'm not really sure how you can gauge that sort of thing objectively with ratings. I believe somebody offered the idea of ditching the Mature rating and making all Mature games Adults Only. I think that's a pretty good idea, for then the parents REALLY have no excuse. As for whether they're mature or not, I think that something as complicated as that does not distill into a ratings system very well, and should best be left to the reviewers. After all, the only people who'd care about that sort of thing are the ones likely to read reviews, aren't they?