Monster Vault: Threats to the Nentir Vale the greatest book ever published under the Dungeons and Dragons, 4th edition moniker. While my feelings at this point are much less enthusiastic, I will still readily declare that it is a fantastic book. By the title, one can identify that it probably is in line with the "Essentials" line of products, which to many people will probably make it sound next to worthless. While I do have my own reservations about Heroes of the Fallen Lands and Heroes of the Forgotten Kingdoms, I do not believe that wholesale disapproval of the Essentials line is justified, since there are a few products (this included), that stand out, even above many "non-Essentials" books published over the last few years.
The title of the book suggests both that it is a sequel to the first Monster Vault (which it is), and that it will focus primarily on the Nentir Vale (the default setting for Essentials). Unlike the first Monster Vault, it does not come with a prewritten adventure which utilizes the monsters contained within, but instead takes a more organic approach, which I think is much more successful. The book begins with an introduction of some of the "powers that be" within the Vale, and then goes in to descriptions of the sorts of monsters found there (to build upon the first Monster Vault), and, in doing so, takes a narrative approach to describe why and where these monsters might be encountered within that setting, including often multiple adventure hooks to go along with them.
There are a few monsters in the book that seem a bit "tacked on," in that they initially seem inappropriate for the locales of the Vale, but through narrative description, their presence is justified. Many of these "tacked on" monsters were classics or peculiar ones from previous editions that had not yet been statted out, so their inclusion in the book I can understand, but the fact that the authors went out of their way to try to coax them into the narrative of the rest of the book made them really believable and appropriate. Foremost among these, for example, are the Cadaver Collector and the Penanggalan, which are pretty out there as far as creatures are concerned, but with the descriptions one can quite easily come up with ways to use them. Still, most of the statted enemies in the book are ones mentioned specifically in the introductory area, so that they are not just vague background noise, but actual, legitimate "threats." So therefore many of the described enemies are factions.
To me, the best part about the entire book are all the factions. Factions are not a new concept in 4e; they've been used, with varying success, in almost all of the location guides (Hammerfast, Vor Rukoth, Gloomwrought, etc.), but I thought that the factions presented in this book were particularly colorful, interesting, and believable. Upon reading about many of them, I immediately had ideas in my mind about how to create an adventure featuring them as allies, antagonists, or even both at different times, as well as ways to transport them "out of" the Nentir Vale into my own game world and use them independently.
This last point brings up another aspect of the book: modularity. While it is ostensibly set in the Essentials world, in the Nentir Vale, there is not one creature, faction, construct, anything, that couldn't be lifted out of its "set" location and used elsewhere. There are a whole flock of creatures who dwell in and around the Witchlight Fens. Got room for a swamp in your campaign? Drop them there, even in the Shadowfell, in the Oblivion Bog.
The dragons in the book take a tack which has been more common in more recent D&D publications. Namely, that rather than ascribing them a color and a demeanor, the dragons are all given names and personalities. Calastryx is a three-headed red dragon; Shadowmire is a black dragon changed by his long residence within the Witchlight Fens. Dragons are among the most interesting (and enduring) enemies in the Dungeons and Dragons universe (they're even part of the name!), and so therefore the recent emphasis on individual dragons, who could be poised to be one-off adventure-ending opponents, or just as easily tier-long orchestrators and more distant threats, makes the seem a lot more interesting, alive, and, most importantly, usable.
A personal favorite from the book are the Felldrakes, low level monstrous drakes magically mutated by the corrupt wizards of Bael Turath to serve the empire and their Tiefling masters. These creatures specifically would feel right at home in Vor Rukoth, and actually I think would significantly improve the playing experience of a party exploring that city. Vor Rukoth by itself didn't seem unique enough; it had lots of locations and lots of adventure hooks, but still nevertheless didn't feel very populated, since there wasn't any time spent really getting in to the sort of things that had taken up residence there. Adding in marauding Felldrakes, gone wild from being abandoned for so long, would be a great looming threat for adventurers exploring the ruins there, and would be a great go-to creature to throw in to just about any encounter that needed a few extra jaws to chomp on the good guys. While the Felldrakes themselves are only levels 1 to 4, there are special ones described, Dark Drakes, that go up to level 8. Combining them in different ways and scaling the numbers up, one could pretty easily come up with felldrakes of any heroic or low paragon level.
Monster Vault: Threats to the Nentir Vale is probably the best Essentials-keyed product on the market, and even a strong contender among all of the monster manuals among the 4th edition products. Oh, and it comes with a two-sided fold out map (as everything seems to these days), and 8 sheets of die-cut tokens (ditto). All of these things are enclosed in a sleeve, rather than a box, so you have to be careful if you want to keep everything together to squeeze it well while you're taking it off the shelf, or else things might slide out the bottom. I could dock points for the packaging, but I'm sure they expected most people to throw the map in whatever box they have that has all their other maps, pop the tokens out and throw away the sheets, and to just file the book on the shelf. I don't use tokens, so I just keep all of it together for propriety's sake.