Pretty interesting debate about what makes a game mature.
I agree with naznatips on the gameplay. It should be challenging, unique, and immersive. However, I'm a fan of a deep and complex storyline, and characters I actually want to care about. I find the MGS series to be the best combination of gameplay/storyline/graphics, and hence its my favorite franchise.
However, as naznatips also plunged into, graphics don't make an experience mature by itself. As long as it provides a good challenge, and in some instances just incredibly fun, it can be leagues better then your average button masher.







