The Monster Manual has been reviewed in-depth by several sources already, but I received my comp copy from Wizards of the Coast a little late (I'm sure it was just an oversight...) so I'm only just now getting to my own review of the world's most famous tome of monsters -- with the exception of The Monster Book of Monsters from the Harry Potter universe. Good news: The Monster Manual will not try to eat you. It may try to eat your players however.
* Keeping it simple: The first rule of The Monster Manual is that it's not going to try super hard to encompass every single variant of every single monster. Is a monster proficient with its weapons (page 9)? Yeah, sure. Are we going to list its armor and equipment? No we're not, and who would want stinky monster equipment anyway? This fits nicely with 5E's approach of keeping things simple.
* High fantasy with a touch of humor: The artwork ranges from high fantasy-style watercolors to little sketches and doodles. There is no joy quite like seeing an otyugh galumph along at high speed (page 8), its tentacles streaming behind it like a dog's ears. Also, all the women are clothed, including repeat offenders like the marilith, dryad, and succubus (an entire film has been dedicated to the teenage boy fantasies ignited by the sight of a naked succubus in the original Monster Manual).
* Legendary monsters: Many of the monsters are reimagined, and others have been given a place as legendary monsters that change the terrain and have special powers in their lair. Aboleths, beholders, demiliches, mummies, unicorns, vampires, and dragons among others. Also, the tarrasque.
* We call it Dungeons & Dragons for a reason: Dragons take up a large chunk of the book, as well they should. They're not quite as systematically catalogued as earlier versions, but they're pretty thoroughly explored. Also, dragon spells are optional.
* Goblinoids will kill you: This was a harsh lesson from the D&D Starter Set. Bugbears get surprise attacks that inflicts an additional 2d6 damage if they hit first (surprise or winning initiative), which compounds the awfulness that is being surprised. Goblins can Disengage or Hide as a bonus action. Kobolds have pack tactics that gives them advantage on attack rolls when fighting near their allies. Orcs get a bonus action to move their speed towards a hostile creature. Hobgoblins are like a combination of the bugbear and goblin -- they do additional damage when fighting near their allies.
* Names have been streamlined: No more ogre mages, they're just called oni now. Bar-Igura demons, which apparently were hard to spell for some people, are now Barlgura. Titans are now called empyreans (because I guess titans are associated with creatures even more powerful, if the Tarrasque's title is any indication).
* Odd monsters have been updated: Remember merrow, AKA aquatic ogres? Now they're a unique monster. Piercers are back (having previously been replaced by the darkmantle, who is here too) as larval forms of ropers. Psionic monsters don't have special rules, they just inflict psychic damage. Will-o'-wisps are now undead. Wyverns have hooded heads like snakes (not fond of this change). And most paralyzing effects (carrion crawler, chuul) allow a save every round -- except for the homunculus, who can knock a character unconscious with its poison for up to 10 minutes without an additional save.
* Crazy outsiders are back: There's the obligatory sections on demons and devils, with lots of fluff about how they work but not a lot of mechanics (possession? demon amulets? Make it up DM!). But there's also a bunch of guys from the original Fiend Folio and Monster Manual II who are back and better than ever: gith, modrons, slaads, and yugoloths. Welcome back guys, we missed you.
* Unloved misfits are back too: A lot of monsters that were in the original Fiend Folio are also back: aarakocra, giant bat, bullywug, blood hawk, bullywug, death dog, death knight, drow, ettercap, flumph, grell, grimlock, hook horror, kenku, kuo-toa, lizard king, mephit, ogrillon, quaggoth, revenant, scarecrow, and shadow demons.
* Throw out your miniatures: There are miscellaneous creatures in the appendix, including mundane animals and not particularly special monsters like axebeaks and dire wolves. It also includes every critter renamed "monstrous" in 3.5 with their names changed back to their original roots as "giant." This means there's much less variation in the monster sizes -- no colossal centipedes (that size doesn't even exist in 5th Edition anymore). Giant spiders and giant lizards are now large. Flesh golems are medium. Bone devils have wings again. Glabrezu are now large. As a result, many D&D miniatures no longer accurately represent their monsters.
* And NPCs too: There are 21 NPCs of varying levels as filler. There's not a lot of flexibility here in how the characters are created -- it's more like "name level" characters from AD&D in which you pick a character name and that vaguely represents his or her power level (so a "veteran" is more powerful than a "thug," a "cultist" is weaker than a "cult fanatic," and an "acolyte" isn't as powerful as a "priest"). There's some stuff to work with here, but the building blocks of characters won't be available until the Dungeon Master's Guide.
This Monster Manual is the crowning achievement of all the monster manuals that have gone before. From the artwork to the random quotes about each monster to the curious arcane details about the monster ecology, this manual knows that what makes a good monster book is one that strikes a balance between stat blocks and story. The 5th Edition Monster Manual isn't perfect, but it comes awfully close.