My favorite part of the book is the exploring the underdark section and the geography section. It has a good geography map and nice descriptions of the various cities and places in the Underdark. It has a good description of the different layers of the Underark, Upper underdark, Middle underdark, lower underdark and the types of flora and fauna present. A lot of this is rehashed from prior editions, but it is still a very good description.
The new feats, PRCs, spells, monsters, equipment, and magic items are typical, nothing special.
Earth Nodes is a type of magic I havn't seen in the FR before. You can get multiple feats to allow you to cast more powerful spells near an earth node(increasing your caster level by the power of the earth node).
They also have stats for NPCs from Menzoberanzan, Mantol Derith, Gracklstugh, etc. There are some spoilers for War of the Spider Queen if you havn't read book 3 yet, or if you are playing the City of the Spider Queen module and are still in the first couple of sections of that adventure.