First off, the term "Mature" campaign does not indicate R rated material. Get your minds out of the gutter! I've been in the gutter a long time and it's getting crowded here.
There has been a bit of talk about the high end of D&D recently. Having played my share of high-end characters and campaigns, it's got me thinking. High level campaigns are really the continuing extention of an established game in which the characters have progressed to this stage of game play.
I don't particularly subscribe to the concept of a "Level Cap" at high level games. It has been my experience that each gaming group discovers their own level of complexity where the mechanics themselves make further progression prohibitive. Some of these games reach this point in the late teen levels, others well after level twenty. But my point is here, is that an artificial "Level Cap" works against the goal of making the new edition flexible to as many groups as possible.
What I'd like to discuss here is the "campaign" part of the High-Level Campaign equation. As characters progress through the story arcs of the campaign, and the players become increasingly familiar with the rules that govern their character's abilities, a shift takes place between the Players and DM. With increasing resources and greater personal might, Characters are far more able to pursue their own agendas and destinies. And in so doing, the players can shoulder a bit more of the burden for running the campaign from the DM.
As a DM, it is very helpful to start handing over limited campaign control to the players at this stage. Players of high level characters set their own plots into motion by their very actions. Changing the very landscape of the game, and in a way writing the seeds for their own further adventures. Not only can this provide fodder for the DM to design proactively, but it can by the players' own design, offer adventures which the DM can design reactively. Forging new Kingdoms, Crusades, Syndicates, and Arcane Counsils, all never happen in a vacuum. And through these plans, the campaign can change in ways that affect future groups of adventurers.
I'd remind those of you reading that many members of Greyhawk's fabled "Circle of Eight" were at the beginning, player characters in that campaign.
Which leads me to another aspect of the High Level Campaign. With the players' characters building a lasting legacy, this could become an opportunity for one of them to run a few adventures of their own as DM. The High Level Characters evolve into patrons and mentors for a new generation of characters. For there are always challenges to the High Level Characters goals, and some of those challenges are too widespread or too petty to warrant the direct attention of these powerful heroes. The High Level Campaign itself evolves slowly into a shared world, where the actions of waves of characters weave into a living, nuanced setting. And the original DM gets the rare opportunity of walking through their own creation and creating for themselves personal legends first-hand.