Some great ideas being hashed out in this thread...
Andrew Rilstone makes some excellent points. The definition of "maturity" and how that definition has become pigeonholed rather than explored more thoroughly (dare I say "exploited?") seems to be the crux of the problem. Of course, everyone has different ideas about what makes the most entertaining game, and even people who seem to agree on the surface can find that a detailed conversation reveals some key differences that would never mesh at the same table.
For my own part, I've always striven to develop my games with an eye toward individual choices. No force-feeding, hand-holding or predetermination mar my scenarios if I can help it. PCs, and even NPCs, make their decisions in the moment and take into account as much information as they have available to them at that time. Don't get me wrong, I like "Good vs. Evil" and "struggles with potential worldwide consequences" but only if the choice is made by the PCs to become involved in a story at that level.
I think that anyone who has picked up a copy of the Questus - The Whispering Woodwind adventure is familiar with the way in which in allow for but do not force mature roleplaying in my scenarios. As it pertains to game design in current and future CMG products, I use a fairly simple graphic representation of the mindset of NPCs called the CMG PROSE System. Coupled with an overview of the situational circumstances, it allows for a DM to make decisions for the various NPCs on the fly to create a cohesive and believable scenario...even when it is quite likely no two DMs will do it the same way.
From my perspective a "mature" game presents opportunities for play without fashioning them in such a way that a simpler game can't also be enjoyed should a game session move in that direction.