Chapter 11: Encounters
Nothing in this chapter is really new or optional. The DMG starts off with planned/triggered encounters, then moves into random encounters and how to set up Random Encounter Tables. There's also a nice little chart telling you when to check for a wilderness encounter.
In both the PHB and DMG, the Encounters chapter is where surprise is covered. The DMG includes a chart of modifiers and encounter distance. In addition to this, there's the infamous Encounter Reactions table that many DMs hate.
Chapter 12: NPCs
This chapter is just a basic description of what the different types of NPCs(hirelings and henchmen mostly) are and whats expected of both the NPCs and the PCs when using them. Despite the advice against having magic shops, this chapter also includes an NPC spell cost chart. Resurrection isn't even on the list, and Raise dead requires not only payment or service but ALSO requires the character to be raised to be of similar faith and belief.
Chapter 13: Vision and Light
This chapter gives out a lot of fairly useful information. Where Chapter 11 may give you encounter distance, this chapter lets you know the various visibility ranges(in the PHB). It also gives a chart that gives the radius and duration of the various light sources characters have a tendency to use.
One big note in this chapter. The fight over what Infravision IS has plagued the various editions of D&D that have used it almost from the beginning. The Core 2nd edition definition of Infravision is that it's basically d20's magical darkvision. It's not heat vision.
Tomorrow: Time and Movement, and you thought you knew the rules for running!