I think Mature is mostly looking at a particular topic from an in depth, intellectual and perhaps philosophical perspective. As such, almost any topic can be portrayed as mature, I feel. A hypothetical example I often use in threads where this topic comes up is an in depth look at a friendship between two characters that has its ups and downs based on a variety of external and internal events, in what ways that friendship is positive and in what ways, if any, that friendship is actually a source of negative.
I do feel it's gotten to a point where there's groups that expect "mature" to basically mean uncomfortable topics. Whenever I see a topic about mature themes (on any forum, not just ours), I can typically predict that that means the topic will be mostly about violence, sex, rape, incest, racism, and sexism. These topics are often dark, but I also often find they are often don't really make a story any more mature simply with their inclusion. That depends on how their inclusion is handled.
I also think that for some people, explicitly seeing it (rather than just knowing it exists) is required as a form of shock. Almost a slap in the face of "yup, this is mature." If all the sexual underpinnings of Game of Thrones was removed from explicitly being on film, but we knew it still happened, does it really make the story less mature? I don't think so. But I think there are definitely people that disagree with me. If rape exists but we, as the player, don't actually see it, then it feels muted and in the words of some even unrealistic or sheltered. I always find this perspective somewhat startling since I've never seen such acts occur in real life, though I know they exist and I recognize them as being bad things. Perhaps this is a statement of our ability to disassociate our reality from those we experience in media.
I'm definitely not someone that needs to see an act like that to feel it's a requirement of being mature (or even dark), really.