Honestly, there are some days where I wish these "mature" games didn't exist. Video games were seemingly more fun before blood and gore, breasts and the overuse of the word "fuck" became big deals. As an old man who grew up playing the Atari 2600, ColecoVision and the NES and continued all the way to the PS3/Wii/360, I'm torn between the two eras. I long for more games like Uncharted 2, which combined the two eras to near perfection.
 
For all the praise Heavy Rain gets, I don't think David Cage gets enough scorn for his need to have sex and nudity pad out a mediocre story line and uneven acting.

Mass Effect and Heavy Rain are certainly examples of where I'd like to see storytelling in video games go, although I strongly disagree that Half-Life is an example of an immature video game. I also don't agree that games which aim to make the player feel powerful or games that depict violence can't be "mature". In many ways I feel that when you talk about maturity you're actually talking about depth and complexity and we must remember that video games aren't all about the story. While games will hopefully go the way of Heavy Rain and Mass Effect in terms of narrative, gameplay is what lies at the core of the game and most developers are not only going to perpetuate the use of gameplay elements that are deep, complex and enjoyable, no matter how they effect the story, but they're also almost always going to place gameplay ahead of story elements.
 
Finally the age ratings aren't facilitators for developers to convey deep and intellectual story concepts, they are there to restrict minors from playing games with explicit content and the real problem with the AO rating is not the stigma attached to it or that many retailers are reluctant to stock AO games, it's that no console manufacturer will allow AO games on their system meaning that instead of being a mature rating, the AO classification is effectively a ban.

Video games are still evolving at such a fast rate that the main goal of every developer is to make a fun and interesting game no matter what it is really about or what kind of game it is. The focus, although sometimes on great stories, doesnt go any deeper than that. It is way more important to come up with new game mechanics and designs rather than story or other more meaningful content you would see in a movie or read in a book. Right now, an M on the rating system only means "Kids shouldnt see this cause its pretty bad stuff that a kid couldnt really handle the right way", and thats all. Basically, the maturity your talking about is a different perspective, or sense, of maturity than the one the ratings people are using, if that makes sense. I think it will be a while before we start seeing games with content only adults will really understand that isnt just boobs, sex, and crazy insane murder.

AO games aren't that great. some M rated games shouldn't even be M rated. like halo for example. there wasn't really all that much of bad things in the game. sometimes there would be blood but it really gets it's M for the multiplayer. the multiplayer is where you hear a lot of the swearing in the game.

I think you can abstract maturity further than Heavy Rain. When you think about it, the maturity of the concepts and depth required in a game like Pokemon far outweigh that of a game like Gears Of War. It's the difference between mature content and mature ideas.   
 
Then you have the whole other argument in which we talk about the maturity of game design. Halo, for example, is not a particularly mature game in itself, but the way the series has been developed, the mechanics and the understanding Bungie has of game design has obviously matured since the franchise' inception.
 
Sometimes you want to watch The Fountain, sometimes you want to watch Transformers 2. Despite the fact that The Fountain is a great film, it is also fairly heavy to digest. Sometimes it's nice not to worry about that stuff and just watch giant robots fighting.

Fuck Heavy Rain.  It wasn't so much a mature game as an incredibly pretentious one.  The narrative (poorly crafted as it was) did nothing new in terms of mature gaming.  The themes it touched on were handled much better by Silent Hill games.   That Heavy Rain involved adult themes is easily undercut by how poorly it handled them.  How one can knock the inclusion of sex as immature in most games yet hail the childish use of sex in Heavy Rain as mature is beyond me.  
 
I think people confuse maturity with depth.  Custer's Revenge, Rapelay, Heavy Rain, and Ninja Gaiden II are all mature.  It refers only to including adult content.  The label says nothing of sophistication or gravity.  There's nothing immature about violence, gore, sex, or obscenity.  
 
Conker relished in tacky humor, but the execution was great.  They were capable of doing justice to their intentions.  Heavy Rain, Braid, and similar games try to pawn themselves off as deep by virtue of serious themes, but they almost never do anything interesting with those themes and cannot handle them without self-serious brow-beating.
 
To no small degree, video games are a very limited medium.  For the most part, esoteric, deep, or intellectual elements don't translate well into games.  You have to balance the material with fun gameplay, and that limits appropriate material.  Hell, Heavy Rain is hardly even a game (more of an interactive movie), and it still couldn't manage.  
 
And, finally, Custer's Revenge is not a porn game.  It was a bit of lewd humor that was poorly made.  It was not designed or played for titillation or sexual gratification.  There's a big difference.

Yeah 
 
Just because the word fuck is in the game it doesn't make it mature.

The existence of one doesn't have to impact the existence of the other. 

I didn't really like Heavy Rain, there I said it.

 Making mature games has been a task that this industry of ours seeks to understand with a constant pace.

There have been several games that have been seeking that glory filled valley of mature games, from Custer's Revenge to Heavy Rain. The have tried many ways to make mature games for gamers that are getting older. There is even a rating for mature games and a rating for games aimed at adults. The only problem is that AO games have become synonymous with porn video games, games such as Rapelay. Is the problem with the distinctions between the mature games and the games that get put under the banner of the AO games which invariably get put behind the retailers desk, or more common just refused outright to be sold, or even played on the consoles of the time.

  Custer's Revenge was a porn game nothing really else, the point of the game was to get from one side of the screen to the other side of the screen so that you could rape the Native American woman tied to a post. That was the entire game just do that over and over, not exactly what I would call going forward for the industry. Makes you glad that our industry has matured since that day doesn’t it? If only however we matured more, while the game graphics and structure and story have advanced we are still children trying to figure out how to properly use our industry to say what we want to say.

  Another such game that is trying hard to bust into the area of mature game content is Heavy Rain, a game centered on a story about child kidnapping and what one father will do to try and save his son. The story is a very mature one in an industry almost centered on space marines and nerd wish-fulfillment ala Half-Life. Games that want the player to feel strong and powerful against opponents that they normally wouldn't be able to face, being able to save the world, or the princess, in the process. These are not mature games in the sense that the older gamer generation wants the games to become. These are games that revile in blood, gore, and sex games that want to be taken seriously by other mediums while allowing you to kill hordes upon hordes of enemies in minutes.

  Do not get me wrong, those games are some of my favorite video games, however these are not mature games not in the sense that movies have mature movies. Heavy Rain on the other hand gives a very mature setting and story along with a nice set of consequences for your actions. I believe this is how the medium will become able to really use the term mature game. What really makes these games mature is the story, the sense of vulnerability in your character, and consequences to your actions. Such games do not even need to be constricted to Heavy Rains formula, take Mass Effects game play; it's all about the action when something happens. They want you to move about to shoot when you want to shoot to stop when you want to stop. They want you to inhabit this character and this universe. This is exactly what they are creating for the players, a universe constructed by your decisions and give you the ability to change the galaxy to whatever you want it to be become, or find out that the decisions you chose have more far reaching consequences than what you thought they had when you first decided to kill that Rachni.

 This is what I believe the more mature games will soon steer more towards, consequences and storytelling will soon become more popular and not just in RPG games. Here is hoping that soon mature games won't necessarily mean there are tits and someone's head is getting ripped off.