Kingdom Come is where the Iron Age comes to an end. By Alex Ross and Mark Waid, KC takes as its main conflict the mores of the previous ages and how those do not work with the "grim 'n gritty" of the Iron Age.
The basic plot is that the older heroes are pushed aside in favor of younger, edgier, more violent heroes. However, when a line is crossed, Superman returns to set things right. The problem is that the Man of Steel's intervention may just ignite Armageddon.
In certain ways, KC is a political book, not as overtly political as Ross's Project: Superpowers, but it's certainly there. There are several factions in the book, and they all believe that they have the right means to save the world. Of course, resolution only comes when all the factions come together and put aside their differences. In this way, I think that KC is even more relevant today than books like Watchmen or DKR. There are many groups in America (and around the world) that think they have the right of it. In the U.S., you have the obvious liberal/ conservative dynamic. The thing is, both sides have good ideas, and both sides ultimately want to see America prosper. Kingdom Come illustrates that peaceful coexistence and partnership leads to more fruitful endeavours than constant bickering and struggle. Compromise is not defeat, and this book really drives that home.