First overall impression; this is without a doubt a book aimed at new DMs, rather than a reference book or book of options aimed at experienced players. That doesn't mean that there isn't anything of value for more experienced gamers, there is, but the emphasis is clearly on holding newer DMs by the hand and teaching them how it's done. That approach, I am certain, is not an accident.
Aesthetically, it's a beautiful book. There is a ton of artwork throughout, with varying styles, which is something I really like and which hearkens back to the original AD&D books, which had art by multiple artists. It ranges from the creepy to the heroic to the downright silly (I refer especially to the "chibi modrons" on p. 66, accompanying the description of the plane of Mechanus). The pages all have a faux parchment background, but I didn't find it a distraction and it doesn't seem to interfere with the text or diagrams, as some similar things have done in past books.
Specific references to published campaign settings like Greyhawk or Krynn are few and far between, but are definitely present, and even 4th edition fans are thrown a bone as the Dawn War pantheon (the default pantheon of deities for 4E) is used as the sample pantheon in the "Gods of Your World" section. They are covering all the bases.
There are sections on campaign events, a few options for evil characters, random downtime tables, and detailed rules for a more grid-reliant combat system that includes facing, flanking, etc. We finally get random encounter tables based on terrain and CR (although they're not proper encounter tables from a 1E point of view; they're more lists of creatures, and would need the DM to turn them into roll-this-get-this-encounter format), descriptions of the planes, and of course we now have extensive lists and descriptions of magic items (I haven't done a full comparison, but it looks as if nearly every item is illustrated, which is a nice touch). There are also treasure tables which are tied to CR.
But the star of the book is the advice for new DMs. From guidelines on tailoring adventures to the tastes of your players, to creating campaign worlds (including pantheons, mapping, settlements, campaign events and when to use and not use them, different flavors of fantasy such as sword and sorcery vs. mythic fantasy, how to create adventures (including different types of adventures such as wilderness, dungeon, mysteries, etc.), how to create specific encounters (and a nice overview of random encounters and why they can be useful - yay!), motivations for villains, mapping dungeons, and how to stock dungeons. There are even some rules and guidelines for dealing with crossing genres (there are rules for using alien technology, for instance, and firearms). All that is nice for experienced DMs to review, and often comes with handy tables (although they must perforce be somewhat generic), but the intended audience is clear.
On the whole the 5E DMG looks to be a very good book. Experienced DMs will find a lot of useful tables, and the magic item and planar descriptions will be especially useful. Some DMs will find the enhanced combat options indispensable, while most will be able to turn the CR encounter lists into meaningful encounter tables.
But beginning DMs will find this an enormous resource, and will be well rewarded by a close cover-to-cover reading. It goes far beyond the "what is a roleplaying game?" introductory material into the nuts and bolts of campaign and adventure design, as well as resources and guidelines for running a game at the table.
Another solid rulebook for 5th edition.