Angels in America ends on a note of hope. Not in a super-cheesy everything-is-awesome-and-perfect-now kind of way, but in a yeah-things-are-tough-but-life-is-still-beautiful kind of way. As Harper says at the end of Act 5, "In this world there is a kind of painful progress" (5.9.2). Though the play definitely highlights many of the problems with America, the planet earth, and the entire state of the universe, ultimately it is optimistic. The play isn't saying that everything in the future is going to be easy, but that we are slowly moving toward something better.
The end of the play, especially the epilogue, is pretty much an argument for the necessity of political progressivism. Louis spends the whole epilogue talking about how awesome Mikhail Gorbachev is for instituting perestroika, a set of economic and political reforms that ended the Cold War and broke apart the Soviet Union. Louis says, "Whatever comes, what you have to admire in Gorbachev, in the Russians is that they're making a leap into the unknown" (Epilogue.7). Louis admires Gorbachev's courage to make major changes even though it's not totally certain how it's all going to come out.
(For more on Gorbachev and perestroika, check out "What's Up with the Title?")
Prior's last lines pretty much sum up the progressive view of the play:
The world only spins forward. We will be citizens. The time has come. Bye now. You are fabulous creatures, each and every one. And I bless you: More Life. The Great Work Begins. (Epilogue.31)
Notice that Prior uses the phrase "The Great Work Begins." This is the angel's favorite thing to say to him whenever she appears, and it's how Part One: Millennium Approaches ends. What's interesting is that, though the same sentence is used to end both parts of the play, the intention behind it changes. The angel's idea of "The Great Work" is essentially conservative. She wants human beings to stop growing and changing. Earlier in the play, though, Prior rejects this idea, saying that such a thing is impossible. It's human nature to want to migrate, to learn new things, to improve ourselves. To Prior, "The Great Work" seems to be the "painful progress" that Harper speaks of (5.9.2).
For more on the end of the play, check out "What's Up with the Title?"