It gives suggestions on how to tell if the house rule is needed, explains some of the concerns to watch out for.
It should bring an end to some of the "why do you ever need to house rule" sentiment, or some of the people I've seen who get really bent out of shape if anyone suggests they're going to house rule. Not only does the new edition recognize that people may need to house rule, it apparently tries to facilitate it to some degree.
It also has a couple sample house rules, which for all intents and purposes could be considered optional rules. I would guess a lot of groups will be using them. Fumbles, critical skill check successes/failures among them.
The DMG is primarily a common-sense primer on how to DM. It's not really a reference book, and the amount of useful reference is very small. The town of Fallcrest and the Nentir Vale are a very nice inclusion, providing an adequate base of operations and surrounding environs for a starting campaign. This and the information on modifying and creating monsters are the DMG sections that will be of the most use to me.
Monster creation looks to be less time spent doing tiresome math and more time spent choosing/designing cool powers. Template application looks to be quite easy, with every class having a template. Monsters are leveled before the template is applied (no class levels anymore). This section made me want to convert some old faves that didn't make the 4E MM cut.
I guess I'm not the only person who thinks the DMG looks useless for the most part. ^_^