Probably not an appropriate topic for the Christmas season, but it's something that's been on my mind lately.
For one thing, why take Oz dark? Why not simply make it more realistic or consistent? I think it's because people tend to view Oz as a bright, pleasant place and the easiest way to create a contrast is to go all the way dark.
Another question that is relevant to ask is: What makes something dark? For those of you who don't know, it takes more than vampires to make something dark.
Darkness in fiction (and gaming) has to do with the role of evil in the story. If evil is winning, or has already won, it's dark. If the heroes live in the Good Kingdom and fight against the armies of the Evil Kingdom next door, you've got a fairly light situation. If the heroes live in the Evil Kingdom, and are rebels against the Evil King, then you've got darkness.
Both Caliber Comics and Disney produced a dark version of Oz by allowing it to be conquered by the Nome King. While this technique is effective, and lets your favorite Oz characters become heroic freedom fighters, it's a pretty severe change to the setting.
Another way to "darken" a setting is to apply a "dark mirror," taking things that are normally good and making them evil. A dark mirror Oz may have the Wizard, or Ozma, or Glinda in charge, but they are dark versions of their literary selves, harming others for their own self-gratification. Wicked uses this technique, casting the Wizard as the antagonist and Elphaba as a crusader for Animal rights.
With all this dark Oz stuff going around, I'm wondering if I should try and come up with a supplement for AiO with advice and suggestions for running a dark campaign.