First, let me be clear and say I'm not exactly sure what people mean when they say something or other is "Mature". If by "Mature" you mean realistic, then I don't see the big deal. If we're using "Mature" as short hand for complex, that makes more sense, but why not just say "Complex"?
I've heard some people say that a "Mature" game was one that didn't hold your hand, but that doesn't make sense. Every game (Most every well designed game at least) holds your hand to some degree, better games just know how to do it in an unintrusive way.
A game that doesn't treat like an idiot and takes itself mildly seriously isn't a mark of greatness, it's just competence.
So I'm gonna go with complexity, because that's at least positive. (Spoilers may follow, and this won't make any sense unless you've played the games anyway)
The Witcher games have been a series I liked but never quite loved. It's true they're highly above average, but they've always skirted the edge of greatness for me.
The stories are well written but I've never found them particularly profound or evocative, all I can say is that they're good, but I've never made any kind of personal connection to the characters or themes (And I'll get to those later)
The use of choice is inspiring, especially in the second game, I'd really like to see more games make use of these techniques. But like I said, the story itself never really got to me.
For example; in Mass Effect, my choices have less impact on the story, but a more meaningful impact on me personally, this is because when Mass Effect gives you a choice it's not about what makes sense, it's asking you an emotional or philosophical question.
In The Witcher, your choices affect the plot more, but never caused me to question what my goal was, I always knew what I wanted, the question was just how to get it, which only proved difficult once or twice. The Witcher has a lot of characters and situations that are morally grey, but not always to a positive effect; most of the time I found myself not struggling between which choice was less despicable, and instead just not giving a fuck either way because there was no real preference to be had; each of the conflicting organizations were equally reprehensible, neither choice would lead to anything resembling a desirable outcome, so I just don't give a fuck, I hope they kill each other and rot in hell.
This problem might have been mitigated if I connected the characters personally, but I never did, to this day I couldn't tell you anything personal about Garelts friends beyond where they stand in the conflict of interests; either they aren't complex enough to get invested in or they never share their motivations with Garelt. The closest I ever got was Iorveth, who I found walked an interesting balance between being despicable and sympathetic; it's clear he's an extremist, but I understand why, and he thus helps to understand why people take up extreme stances. But I still never really got to know him personally, and most of the plot doesn't have much to do with him.
I also found Letho interesting, and as it turns out he's the character I most identified with, because, like me, he was just concerned with survival. The way he talks about his actions makes him sound totally blameless and I love that, because it illustrates how dysfunctional Tameria is, and it was hard to blame him for just moving things along in the direction they were going anyway to ensure his well being. Unfortunately you don't really get to know him until the last 5 minutes of the game. (Here's hoping we see Letho and Iorveth in Wild Hunt)
The gameplay in The Witcher 1 was between boring and decent; The combat was rather clumsy, but visually pleasing, but the whole hunting thing was needlessly tedious. You're telling me I have to find a book so that Garelt can cut out a fucking tongue?
The Witcher 2 was better; the combat was pretty good at higher levels, if a bit too easy, (Sometimes) but at the beginning of the game you basically had to play hit and run with everything on the face of the earth, even while everybody keeps telling you what an all powerful and mighty slayer of men you are; despite all this you can barely fucking block and you spend your fights circling your enemies Painkiller style.
The monster hunting was better in that you could research your prey by, you know, killing it, but there were still those fucking quests that expected you to know a specific recipe for a bomb (That they don't tell you you need) before you could just blow something up. It's not like you have control over the primal forces of the universe that you use to break down brick walls or anything.
In terms of complexity, I would hold that games like Dragon Age and Red Dead: Redemption are more thought provoking than The Witcher, because while The Witcher games are darker, what is the purpose of them being dark?
What is The Witcher fundamentally about? What sort of questions is it asking? What point is it trying to make? What The Witcher is lacking, I think, is depth.
A lot of people compare The Witcher to A Song of Ice and Fire, but the defining factor of ASoIaF is that it get's you invested in it's characters despite them being, at best, mildly admirable, and it does this by making the central characters directly involved with the conflict. In The Witcher, Garelt is an outsider, he has no reason to be invested in politics or war or social well being, because the game purposely set's him apart from all of it's conflicts. Not only does he have amnesia, but nobody seems to trust or respect Witchers, so nobody commands respect or investment.
At the end of Assassins of Kings, when I heard that the realm was basically going up in flames, all I could think was "Good riddance".
I might call The Witcher cynical, but I wouldn't say it's any more "Mature" than other games of similar or better caliber.
R The games are slightly less mature but still fairly mature and thoughtful all things considered.
