First, read his, because it is pretty much spot on across the board.
All the classics, and as far as I can tell nothing really new. You could, if you know the game well enough, probably recreate the table of contents on your own.
In order to keep things still pretty simple, major league monsters now have two new abilities: Legendary and Lair Abilities. Legendary abilities are basically additional attacks the DM can activate out of turn order, giving them an edge. Lair abilities are a secondary effect associated with the location but also adding additional peril to the encounter.
It's a way of working around how to give solo monsters parity with large PC groups who can dog-pile on a single monster who can only hit back once. Like many things about the fifth edition, it is pretty simple and elegant.
There's no rules for creating your own monsters or upgrading monsters. Common humanoids often have a "boss" version like an orc chieftain.
Gone is the blocky, World of Warcraft-inspired look. Gone too is the eight hundred leather straps. The monsters seem more plain, less topical and more timeless. They aren't as in-your-face cartoony either.
The book is incredibly self-referential. By that I mean it mentions a lot of the literary legacy of D&D. Strahd von Zarovich from Ravenloft appears, as does Acherak the demi-lich from the Tomb of Horrors.
This brings me to perhaps my biggest nagging sense about D&D 5E, namely that when you play, you're playing Dungeons & Dragons. You're not playing a fantasy game, or even your fantasy game. The game is so branded, in its own right.
And this isn't necessarily a crime. When you play 13th Age, you're playing the in the 13th Age universe with its thirteen all-powerful NPC's. When you play Shadowrun, you're playing in the Shadowrun universe. I've often felt like the impact of D&D on the genre of fantasy can not be underestimated. It is like weird literary ouroboros: D&D stole flagrantly from fantasy novels, and then in turn become the genetic code for much of the fantasy literature you see today.
But the game seems so much less like a toolbox than its earliest iterations and more like a pre-packaged product to be played as-is. Yes, you can do it yourself. You can always do it yourself. But that implicit ethos is missing, and must be inferred on its own by creative DM's. And while most of us are, what about the new generation of gamers out there? I'd have loved, LOVED to have five of six blank pages in the back of this book with the heading "WRITE YOUR OWN MONSTERS HERE."