While it's not my favorite topic, or one I have a lot of experience with, I want to touch on "dark gaming." Because people keep thinking that Oz: Dark & Terrible is a dark game.
In my mind, darkness is about evil as the dominant force in the setting. But one thing that people forget is that good is just as important to making a setting dark. Nothing makes us realize how dark it is like a momentary burst of light.
The Oz described in Gregory MaGuire's Wicked is dark because of the domination of the power-hungry Wizard. While Elphaba is an extremist, the fact that she is doing it for a cause that she (and we) believe is morally right, it is much easier to see her as the "good guy" in that scenario.
The Dark Oz comics published by Caliber show an Oz conquered by the Nome King, with a little help from Mombi. That's pretty dark right there. The only real ray of light here is the heroes, trying to free Ozma and restore her to the throne of Oz.
Oz: Dark & Terrible has lots of grey, but damn little true darkness. The Wizard has been transformed into giant animatronic head, but he is credited with genuinely good intentions toward the people of Oz. Glinda is trying to improve the lot of the primitive African Quadlings she rules over, but she's doing it by bringing in her fellow Gillikins to lead by example.
What little darkness we do get mostly comes in the form of Wicked Witches, notably the Wicked Witch of the West and Blinkie, the former Witch of the South. And both of them are simplistic, mustache-twirling evil (not that either of them have mustaches, but you get my point).
The other problem is that there is damn little light. Some character or aspect of the setting that is noble and good could make the whole setting a lot darker, even as it is, just by simple contrast. Judicious use of light could highlight those shadows ominously.