Why does everything that happens in Isaiah—or, all the wrathful stuff, anyway—need to happen? "Sin" is one answer. God needs to punish everyone's sins, hence the Assyrians invade and get slaughtered by an angel, hence Babylon falls and the invaders massacre civilians… and so on and so forth.
But what is sin? It takes all kinds of forms throughout the course of the book, but it pretty much always involves forgetting God, forgetting the origin and the creator of humankind. The Assyrians help God punish Israel for turning away, worshipping other gods, and failing to behave justly (to take care of widows and orphans, in particular). The Assyrian King Sennacherib is himself punished and destroyed, since he openly believes he's superior to God himself. So sin, at its most basic sense, involves turning away from what's really central and important. It comes from getting involved in lots of distractions and egotism.
The first theme was sin, which leads automatically to the second theme: suffering. The suffering inflicted on Judah (and other places) for sinning is pretty significant. Cities are destroyed, people are reduced to poverty and starvation. There's lots of wandering around in sackcloth (not too comfy, gang), plus lots of weeping and wailing. But Isaiah isn't just depicting suffering, he's attempting to show its causes. He aims to show how the injustice of human beings rebounds like a really awful boomerang right back at them.
Justice and judgment—next to, and not behind, mercy and compassion—is probably the biggest theme in Isaiah. God continually visits his wrathful judgment on everyone, constantly, throughout the course of the book. Isaiah basically exists, as a person, to warn about God's impending judgments and to urge people to change their ways before it's too late (which, you know, they mainly don't do).
But God, in Isaiah, isn't mainly concerned with whether people are following all the rites and rituals. He wants to see them put their hearts into their actions, and behave with devotion and sincerity. He isn't punishing people for failing to offer him sacrifices. He's punishing them for only offering him lip-service, and for failing to treat the less fortunate (particularly, widows and orphans) with compassion. He's basically trying to get everyone to straighten up, fly right, and treat each other (and him) with respect. Doesn't seem too much to ask, right?
Two kinds of power are on display in Isaiah: human power and the (so much greater) power of God. Human beings continually strive to use their power to do things like conquer the world (what the Assyrians and Babylonians do), only to see it snatched away from them, proving that God holds the only power that matters. Whenever a human being starts to assert his or her power and make the sort of classically boastful statements that Biblical villains—like Sennacherib—tend to make, it's time for an immediate smackdown. But the same is true even for the small-time criminals described in Isaiah, those powerful people who won't pay their workers a decent amount of money, and cheat them, but pretend to humble themselves before God in the temple. They all get their karmic beatings, thanks to the just power of God.
Loyalty is what God demands from Judah and Israel. They shouldn't get distracted by other gods, or by lust for cash or craving for power. It's closely tied in with obedience, but there's an element of fervor and love tying it all up. At the same time, God's loyalty to his people is also important. God keeps promising that he will, in the end, own up to all his promises. He will guide everyone to a state of peace—even if they kick and scream along the way. People are going to get rewarded for remaining loyal to him. So, you know, it's a two way street.
The big dream that animates the Book of Isaiah is the dream of a future time of perfect peace—basically the exact opposite of what's been going on throughout the course of the book, what with all its examples of annihilation and total war. The wolf and the lamb are able snuggle together, hang out, watch Friends re-runs—whatever. This is also tied up with Messianic dreams, and for Christians, the redemptive mission of Jesus, who they usually identify with Isaiah's suffering servant.
Last but not least in our themes, we have compassion and forgiveness—or, in a word, mercy. These ideas comprise the biggest and probably most important theme in Isaiah (along with justice and judgment). Mercy, in a sense, is what it's all building up to. It's the moment of relief that follows the constant conflict and violence and tension (whew). God might treat his people with wrath to correct them from their ways, but at the end of the day (Isaiah says), he ultimately loves them.
And, apparently, this isn't just a vague sort of love, like the way someone would say "I love Cheetos" or "I love The Sopranos." It's closely compared to human forms of love, indicating how intense and passionate it's really supposed to be. God always goes at whatever he's doing—be it destroying cities or loving everyone—as hard as he possibly can, and in a big way. And—aside from a brief reminder of what'll happen to the continually disobedient people—that's basically how Isaiah ends: with the reign of endless, universal love. It turns out God and Isaiah were both really hippies, in the end, despite earlier death-metal tendencies. Those were just for a historical minute or two, but the rest of eternity is one long jam session to "All You Need is Love."