Spoiler 0: The best thing, Hit Points of monsters is way jacked up over the last playtests. These monsters can take hits! My biggest playtest complaint was that monsters died too easily.
Spoiler 1: and first complaint! There is no List of monsters by Challenge Rating in the book, only an alphabetized Index. So when a player casts "Conjure Woodland Beings", and chooses two CR 2 creatures, it's going to be tough to find.
Spoiler 2: I was surprised at how different a monster's Hit Dice were from its Challenge Rating. For example: an Ogre is Challenge Rating 2, but has 7d10 Hit Dice. I guess since AC and Hit are flat, all the difficulty comes from the HP and Damage. There are monsters that have Hit Dice over 20. There are a bunch of monsters that have stat scores over 20 also.
Spoiler 3: There are swarms! In the animals section at the back, they list swarm of rats, swarm of snakes and a few others. They have Resistance to slashing, bludgeoning, piercing. They do not have any Vulnerabilities. Also, once they have taken damage, I think below half their HP, they start to only put out half their damage.
Spoiler 4: Tons of strong monsters have Legendary Actions. Liches, Dragons, Beholders, and the like, get to act between player turns. What sort of sucks, is all Dragons have the exact same Legendary Actions regardless of color. I was hoping for some of the cool differentiating stuff from 4E, but it's only Wing Buffet, Tail Attack, Make Perception check. I guess Lairs differentiate them, but what if you're not in a lair?
Spoiler 5: There are no gods or named Demons in the book. No Orcus, No Asmodeus, no Lolth etc.
Spoiler 6: I used to have the original plastic toy "dinosaur" set that Gary used as the models for the original Bullette, and Rust Monster etc. I am sort of upset at how the art for the Rust Monster is in the book. It's different, and does not match
Spoiler 7: Lizardmen are the boringest monster in the entire book. No love for lizardmen. Hold your breath.
Spoiler 8: I was looking at the Archmage, and there are several asterisks next to its spells, and it says "considered to be already cast". So it's similar to the 3.5/Pathfinder pre-cast buff spell list, but nowhere near as ridiculous. I think it was Mind Blank and Mage Armor or something else. I didn't see this in many places, possibly only that one.
Can put more later.
source of pic
http://diterlizzi.com/home/owlbears-rust-monsters-and-bulettes-oh-my/