Ok. First off, I playtested BoHM2 so consider that there may be a bias (I don't think so but you have been warned).
The Celestial River Pantheon - most people will either use them or not. Some of the gods are urbane (Enaul & Essoch) and will fit well into a cities or kingdoms type of campaign They are gods of time, future and prophecy. Kulaj is the Earth Mother concept and the patheon head. Urgan is the 'blacksmith of the gods' or artificer archetype deity. Dorana is a chaotic water deity focused on vengeance and retribution (will appeal to the 'dark avenger' types and those who believe in a 'fluid' free wheeling sense of justice. Mallock is a evil deity but rather than the destructive evil, he is the corrupter and the whisperer type of evil. His clerics can fit into any setting, protraying a face of benevolence and helpfulness to the general populous, all the while leading those who fall for the twisted lies and whisperings down the road to hell. Think 'Grima Wormtongue' from the Lord of the Rings movie as a example of a acolyte of Mallock.
The Oracle of Jezer-At
This location can fit into almost any setting. It is a location of a oracle, with the location somewhat detailed out, complete with NPCs. The nature of the site is a great benefit to all matters of divination. All matters of prophecy are discussed and how to handle them as the DM - Omens, Visions, Foresight and Dreams. This is a strong part of this chapter since many DMs have issues with how to deal with the idea of omens and visions and there is much here to help the DM deal with such things.
There is a new skill (Profession: Speaker of Portents), 3 new feats (Create Charm Set, Seer, Vatic Sight), 2 new cleric domains - Civilization and Future, new spells (most dealing with divination and population centres), new magic items and a new class of magic item - The Charm Set.
Underwave
This chapter has NPCs and the layout of the main temple to Dorana. Introduces a new weapon - the Lightning Blade (double sword). Has 11 new feats, about half related to some aspect of vengeance/retribution and the others being 'specials' for the Lightning Bearer prestige class. Has a new domain called Vengeance, new spells and new magic items.
Introduces the Lightning Bearer prestige class - well suited for paladin and militant cleric archetypes. Many will probably find the class powerful and no doubt, some will find the class 'over the top' (Our group playtested and didn't find it over the top but that is us) so there will probably be accusations of 'power creep' (as sometime comes up when people debate Monte's stuff).
As always, people's mileage will vary....
The Fallen City of Enderfel
This is a city that was slowly corrupted into a 'fall' and now is a haven for all manner of ner'do'wells. The chapter has new feats (dealing with corruption and seduction - feats like 'Worm Tongued' for example), a new domain called Corruption, new spells (again, most dealing with seduction and corruption) and the new magic items. Lots of icky 'badness' for those who prefer a seductive evilness to their evil as opposed to the 'kill and maim 'em' evil.
Three new creatures (devils) are introduced. With aliases like Hell's Lawyers, Mattock's Child and Whispering Corrupter, these creatures are best used as subtle, scheming manipulators behind the scenes as opposed to another extraplanar outsider for the party to bash.
The World Forge
This chapter describes the temple area of the deity Urgan. The chapter includes new feats, spells and magic items dealing mainly with artificers. There is extensive rules for making weapons and armor that are more than masterwork but are not 'magical' weapons per se. There is a new cleric domain - Forge and a prestige class: Hammer of Urgan.
The Mountain of the Voice
This is a site of power dealing with the deity Kulaj but can easily be adapted to any god dealing with creation or magical power. Like the other chapters, there is a description of the site and key NPCs. Like the other chapters, there are sections dealing with feats, spells, and magic items. This chapter, IMO, is the most interesting.
The feat in this chapter allows one to apply metamagic to charged items (like wands) at the expense of charges.
Magic Items introduce two new magic item classification - admixtures and keyed spell items. Admixtures are like potions with two or more effects. For example, a Fighter's Cocktail admixtures provides the effect of a bull's strength and bear's endurance.
Keyed Items are cool. Rather than go into them, Keyed Items are available as a free preview on Monte's site.
The prestige class, Speaker of the Divine is powerful. Enough so that I imagine that posts on these very boards will pop up debating whether it is broken or not. It even states that the prestige class is powerful. Our playtest group finds it is balanced but depending on the nature of one's campaign, this prestige class may be too much for some..... YMMV.
Finally, the appendix cover conversion of the material to those who are using Arcana Unearthed in their campaigns.
Hope that covers it for you.
edit: spelling