The Oz of Wicked is definitely a lot bigger and more complex than the "follow the yellow brick road" Oz of the 1939 movie. Oz here is a diverse country with very distinct regions. Maguire took the names of these regions from Baum's later Oz novels, though he dramatically altered them in his own work.
It's interesting that, over the course of her life, Elphaba lives in every major region of Oz. This detail helps to further cement the connections between her character and the land of Oz itself. Though Oz is definitely a fantasyland, complete with talking animals and magic, it also has a lot of parallels to our own world, both past and present. So let's take a look at each region.
Things start off in Munchkinland, which is fitting. It's where Dorothy starts her Oz experience, after all. Munchkinland is an agrarian society with a lot of religious tensions, and the two might very well be linked. Munchkinland is suffering from a long-running drought and increasingly desperate people are turning to increasingly out-there ideas. We can see this with the depiction of religious tensions and their violent results.
Nessarose's birth had coincided with a temporary resurgence of well water in the vicinity. They'd been doing pagan dances and there was a human sacrifice ...The crowd, incited by some rabble-rousing pfaithers and a prophetic clock, fell on him and killed him. A man named Turtle Heart." (126.96.36.199-6)
A lot of the descriptions we get of Munchkinland involve dust and dirt (from the lack of rain) and destruction, as with the torn-up yellow brick road (188.8.131.52). The vandalism of Nest Hardings really symbolizes the overall state of Munchkinland, a formerly lovely place that's on the decline and suffering from outbursts of violence.
The liberated Munchkinlanders were destroying the house. The Witch had no use for frippery, but it seemed a shame to waste a property this way. Desecration was so shortsighted. (5.3.9)
Decline, religious tension, and political turmoil aren't unique to Munchkinland, though. Both the Emerald City and Shiz, our major foray into Gillikin, are sites of religious and political tension. While Shiz bears a strong resemblance to Oxford or Cambridge, university towns with impressive medieval architecture, during Elphaba's time there the city eventually becomes heavily industrialized and dirty (5.6.1).
The Emerald City is a cold urban environment characterized by oppression and fear. It resembles a city under dictatorship, like Soviet Moscow or Hitler's Berlin. Both Glinda and Elphaba hate it at first sight, though for different reasons.
Then, abrupt and decisive, the Emerald City rose before them. A city of insistence, of blanket declaration. It made no sense, clotting up the horizon, sprouting like a mirage on the characterless plains of central Oz. Glinda hated it from the moment she saw it. Brash upstart of a city. (184.108.40.206)
Look at them, Glinda, this is a real question. The Quadlings, having nothing, looked – more – than these." (220.127.116.11)
While Elphaba notices how the Emerald City seems to crush the people within it and make them anonymous, Glinda sees the city as a blight on Oz. Elphaba's critique is social, while Glinda's is aesthetic.
The Emerald City also has some really interesting links to Baghdad. The location of government offices and U.S. Embassies was called the "Green Zone." Rajiv Chandrasekaran called his book on the United States' management (and mismanagement) of postwar Iraq Imperial Life in the Emerald City: Inside Iraq's Green Zone. What's interesting here is that Maguire drew inspiration from news coverage of the First Gulf War when he was writing Wicked (source). If you want to read more about this topic, check out the "Allusions" section.
Aside from being a place of political terror and oppression, the Emerald City is also a place of modernization, as we can see from Elphaba's description above. In contrast to the rural Munchkinland, cities like the Emerald City and Shiz are growing rapidly, becoming more technological and industrial. Such changes can create a lot of doubt, fear, and poverty. Oz overall is a sort of historical mish-mash, a place where old traditions like Lurlineism and the belief in the Kumbric Witch are awkwardly coexisting with the Wizard's "innovative" ideas, like his canal plan (18.104.22.168).
This is what's happening at the center of Oz, but the effects spread outward to the two fringe locations that arguably play the biggest role in Elphaba's development: Quadling Country and the Vinkus.
As a child, Elphaba was dragged around the Quadling swamps by her missionary father, witnessing some terrible things and living in poverty:
"Well, Papa felt his work was farther south in Quadling Country, in the real outback. We had a series of small cramped homes around Ovvels – the Hovels in Ovvels, we called them – that dreary, beastly countryside, full of a bloody beauty." (3.6.7)
Quadling Country was the victim of an imperial conquest. The Emerald City, with its Gillikinese wealth and power, invaded Quadling Country to build roads and mine for rubies, disrupting the environment and killing lots of Quadlings in the process.
We see this same process starting to repeat itself years later in the Vinkus. The descriptions of Quadling Country as both terrible and beautiful are similar to the way the Vinkus is described. Unlike the swampy Quadling Country, the Vinkus is comprised of wide open plains and towering mountains; it's a wild, untamed region.
It's also, notably, the site of Elphaba's personal rebirth after her years as a nun. It's definitely fitting that Elphaba becomes the Wicked Witch of the West and ends her life in the region where legends of the Kumbric Witch live on in popular memory. And the Vinkus is explicitly tied to themes of femininity, sexuality, and magic, even in the landscape itself:
They couldn't help but feel the unsettling eroticism of the landscape. From the eastern approach, the Kumbricia Pass looked like a woman lying on her back, her legs spread apart, welcoming them. (4.1.137)
The regions of Oz, with their geographical, political, and social variety, also represent different aspects of Elphaba's character. It's fitting that she's lived in and experienced each region, and it perhaps explains why the book is divided into geographically-titled sections.
But we have one other setting that bears mentioning: the mysterious Other World of the Wizard (our world). The Other World is only seen in dreams in the book, but from the glimpses we get, we can infer that the Other World is a lot like Oz. The Other World is a place of prejudice (against the Irish, from what we can see), poverty, and hard times.
People moved in short, jerky motions. They were colorless, they were vapid, they were drugged, they were manic. Buildings were high and cruel. Winds were strong. (5.11.34)
It's notable that the Other World is also linked to ideas of the afterlife in this book. There's almost something beyond all comprehension about it, and that frightens Elphaba. Elphaba herself is the rather unfortunate product of the collision that the Other World and Oz are undergoing because of the Wizard's presence in Oz. (It's kind of like the two realities overlapping and colliding into one another on the show Fringe.)