The book has crunch for aquatic combat, flying combat, diseases, poisons, skill challenges, traps, and hazards, artifacts, customizing and creating monsters/NPCs, and guidelines for awarding XP, action points, and treasure. It has a gem of a little section called Actions the Rules Don't Cover with a table that can help you determine how to resolve actions like swinging on a chandelier to knock an ogre into a fiery brazier.
It has a lot of "how to be a DM stuff" that experienced DM's will glaze over or not even look at. It has some 4e specific advice on building encounters that DMs might read once.
It's useful, but probably not necessary for experienced DM's. For new DM's it's critical.