The new Dungeon Master's Guide (DMG) is a very useful reference for new and seasoned DMs alike. Similar to the 3.5 DMG, the 4E DMG is mostly devoted to teaching GMs the black art of creating the adventure and running a game. This may come naturally to seasoned and as such the book is of somewhat less value to those folks.
The book's most easily stated purpose can be found in the title of the first chapter: How To Be A DM. This theme is echoed throughout the book as it goes on to provide reams of inspiration for any DM looking to craft a memorable play experience.
The first three chapters are devoted almost entirely to those folks who are DMing for their first time and want to put their best foot forward. The middling chapters provide guidelines for constructing the actual content of a game: Encounters, Adventures and Campaigns. The final chapters focus on the rules minutae of creating monsters, dealing with environmental hazards and construction of entire worlds. It also presents a completely developed town and area to start your players out in if you so choose. You can also use this area as a useful example when designing your own worlds.
Naturally, however, even experienced DMs would be wise to take a look over this book as it contains numerous useful nuggets of information and guidelines on structuring well paced 4E adventures. It's a great refresher for any DM - especially those who think they know it all. Seasoned D&D DMs are, in my humble experience, usually very thickheaded. They have ONE way they like to run their game and they actively ignore any ideas to the contrary. Every DM has his "way" that he follows like a religion and is very closed off to change or feedback. I am guilty of this myself to some degree, but I try to always remember that I am there to make the game fun for my players. I think some DMs miss these points and these are the type of folks that won't like most of this book.
(FYI, these are most of the people giving this book 1 Star reviews, as they clearly have no grasp of the purpose of this book and are just using Amazon to vent their frustrations with what they perceive the rules are lacking. What these folks are missing is that the rules aren't as important as having fun at the table.)
The bulk of the rules type stuff is contained in the last half of the book. These chapters contain rules on rewarding players, various environmental factors, artifacts, world-forging, monster-making and random dungeons/encounters. For those thickheaded DMs, this is mostly the stuff you will pay for as it constitutes the bulk of what you will need to make 4E adventures. Another interesting thing I personally noticed about the monster creation sections is how easy it would be to "fake" a monster on the fly if the DM needs it. Very cool indeed.
This book's stated purpose is to teach someone how to be a good DM. Truth be told, there are exceedingly few "good" DMs out there and newbie DMs can be advised that if you follow the guidelines in this book then you can't go wrong. With the exception of some seemingly forgotten items like constructing Minions, this book otherwise fulfills it's purpose completely and admirably. The 4E DMG stands as an awesome reminder of what D&D is all about: having fun!