The great thing about Under the Dome, is that the dome doesn't even matter, and, actually, it's obligatory explanation is needless to say disappointing. Thankfully, we can ignore that, because what really matters are the people of Chester's Mill, and boy are there some characters in this small Maine town.
Under the Dome is many things: a study in psychology, in particular psychopathy and guilt, sociology, political science, medical science, ecology, etc. etc. etc. King really outdoes himself I think in the development of the characters of CM, particularly Big Jim Rennie, who by the end of the first chapter, you already love to hate. The dome becomes a magnifying glass, an image with many contexts and many meanings throughout the novel, that magnifies the personalities of the people caught under it. Petty jealousies become blood feuds, small-town ambition becomes megalomania, and addiction becomes faith. The great thing is: this is not science fiction. This is
what would happen if you bottled up 2000 people and kept them isolated from the rest of the world. I find that sad, depressing, but also a bit of a revelation. The ugliness of the world and the selfishness of its people come into focus and turn the good into the blurry background.
I think that Dome has the most layering and meaning of any of King's works I've read thus far, including Dark Tower, which you appreciate and love just for the world that's created, rather than the insights granted. Dome is the opposite; there's no sense of wonder or desire to visit or stay, only the claustrophobic sense of panic, revulsion and inevitability that King rides all the way to the end of this 1000 page beast.