You can break Isaiah down into roughly three parts. The first part (Proto-Isaiah) prophesies death and destruction, but keeps lightening it up with cheery prophecies of a good and holy kingdom at the end of time. He always finds the sunny side. Sure, God might attack and annihilate people for wearing earrings, or appoint children to rule as bad kings over Israel, but he always gets to a silver-lining at some point.
In the end, everyone—God included—is going to be so relaxed and peaceful, that formerly carnivorous lions will be able to eat straw with oxen in peace. A vegetarian or vegan diet will be the rule in the Animal Kingdom (Alicia Silverstone's bringing the brownies, probably). We're not suggesting that the end of days stops with a Phish and Widespread Panic Jam Band festival, but uh, maybe that's one metaphor you might want to try on.
The second part of Isaiah (part of Proto-Isaiah and most of Deutero-Isaiah—the latter of which includes chapters 40-55 or so) is much more revved up and eager to see some carnage. Nation after nation receives prophecies of gloom and doom: Assyria, Edom, Ephraim, Babylonia, Moab—you name it, it's getting the Snacktime Cabbage Patch Kid Treatment: discontinued.
Promulgating the ethos of Iron Maiden's "Bring Your Daughter… to the Slaughter", God hands out beat-downs and dishes out vengeance to all the nations, Israel and Judah included. It's not quite as graphic as some other parts of the Bible, but Isaiah—despite its peaceful rep—definitely portrays a God with a license to kill (and, potentially, a "license to ill" as well).
Finally, the third part of Isaiah (Trito-Isaiah, or chapters 56-66) seems to be coming more from a future perspective—the slaughter is (or is almost) behind us, and now everyone is getting it together, waiting for the peaceful time that the earlier part of Isaiah had prophesied. God's not only going to be the Big Kahuna, but really the only Kahuna on the block, as far as anyone can tell.
The blood and guts fest comes to an end, and everyone will go to God's temple and God's holy people and pay homage and reverence to them, while the corpses of everyone who rebelled against God continue to rot ignominiously in the fields. (Despite this gory little sideshow, the cosmic Woodstock on God's Holy Mountain is really still the main event.)
Also, it would be important to mention some of the Messianic prophecies scattered throughout Isaiah—everyone argues about these (as covered in the later analysis sections, like "Symbolism, Imagery, Allegory"). Is the "Suffering Servant" of Isaiah (particularly prominent in chapter 52) a personification of the righteous members of Israel? Is he Jesus of Nazareth? Is he a figment of a narrator's hopeful imagination? Is he a Messiah yet to come? Obviously, we're not going to attempt to answer any of those questions. This is just to note that those prophecies are some of the big, standout passages in Isaiah, for everyone—in addition to being the most fiercely debated.