It also deals with moral issues. Figuring out if orc babies are inherently evil is a mature discussion. Grey areas 'taint' your standard hack 'n slash adventure that D&D normally deals with, the book deals with those grey things, granted it still assumes a perfect moral good and evil, but it allows for the concept of redemption, so in the above example,
a half-orc paladin stumbles apon two succubi who say that they love eachother, if succubi are inherently evil with no concept of good, they can't experience love, so the pally could slaughter away without any feelings of remorse. But if they can feel love, and are being truthful, then that means they can't be completely evil, so they could be redeemed, so what does a paladin do, she can't just kill them outright. A 'mature' campaign can allow for the second, an 'immature' one never does.
Orc babies - The orc is the standard 'evil force' in many campaigns, in a standard 'immature' game, the concept is almost always never considered, and if it is, orcs would be considered inherently evil. But what if they weren't, what if the only reason they raid is because they were displaced/enslaved by another human kingdom, and so they have developed a view that humans are evil from years of fighting/slavery, when the good kingdom represented by the PCs show up, what do they do?
You could of course use the book without such discussion, without grey areas, but the text is there, moral relativity is there, so 'Mature' it is.
Mature does not have to equal violence or sex, this is an illusion brought on by the MPAA.
I hope the above makes sense, I typed abit to fast, and I do need sleep. mmmm sleeeep.
RX