Our initial inclination was to peg the role of antagonist on the Russian occupation of Czechoslovakia. The threat of the secret police and the oppressive political atmosphere in Prague make the lives of Tomas and Tereza miserable, and are responsible for Sabina's flight form Prague. But then we remember that, through the character of Sabina, Kundera makes a rather explicit point about Communism and the Russians: they're just like any other political system. All political systems are the same, because they are all a form of kitsch. Lots more on why that is, and what's so bad about kitsch anyway, in Sabina's "Character Analysis" and in "Symbolism, Imagery, Allegory."
Well folks, the title says it all: the lightness of being is unbearable. It's a long argument in the making as to exactly why that is, so you should definitely read "What's Up with the Title?" if you haven't done so already.