Pages 6 through 19 of the Essentials DM’s book contain the indigestible rules lump that no D&D manual can really get away from. I don’t have much to say about it, since it’s largely reprinted from the PHB, except that it would have been really handy if there were a table of contents or index in the DM’s book.
One of the few touches of Starter Set flavor is on page 16, which describes conditions: each condition lists a Starter Set character or monster who can impose that condition. For instance, under Blinded, “The goblin hex hurler’s blinding hex attack causes creatures to be blinded.”
I read on a message board someone’s theory, based on a preview of the Essentials Knight fighter not using a mark mechanic, that Essentials was eliminating the Marked condition. In fact, Mark is in the condition list, and the condition’s description even lists a creature in the Starter bestiary that Marks opponents: “The skeleton’s longsword attack marks a target.”