Every discussion I find on MotM is flooded about the player races. My main disappointment with the book is in what will be the new NPC design going forward. 

What are everyone's thoughts in the new style on NPC design? I haven't heard any one who liked it yet but maybe it's because they are hard to find in all the talk about player race design. Even if you aren't going to use the monsters as a DM look at them as a look into WotCs plan for enemy design moving forward.

I'll give my thoughts.

My complaint on 5th ed monsters has always been that they are meatballs with a multiattack and WotC seems to be interested in doubling down on that flaw. If you want interesting enemies it becomes completely on the DM to homebrew or modify.

Now casters are even less interesting with less spells that are individually managed, which isn't less hard to manage then a few spell slots, but the spells they get are generally less interesting damage direct damage spells pushing them closer to manage with a multiattack.

The NPCs not working at all like players creates a further disconnect for me without adding anything interesting to compensate for that cost. 

Force replacement for magic damage really hurts barbarians and a few other things needlessly. It barely feels any simpler from the previous system of magic and non-magic damage with a type so I don't think it accomplished it's goal.

Some people say they'll fix barbarians in 2 years but 2 years of bad monster design is a long time and a lot of books will be balanced for a state some classes don't exist in. 

Overall while I think race changes are fine but boring I think the new monster design is WotC's worst work so far. Just to make barbarians useful or combat interesting if feels like this requires even more homebrew or customization when it comes back in later books.

Other people will have different thoughts though. Does anyone like their new way of building NPCs?

I don’t mind.. if i want a complex NPC that can act like an PC with a custom spell set i’ll create a PC and use it as an NPC.

If i don’t like the NPC as is but also don’t need it to be complex i’ll adjust it to what i need/want in the moment or ahead of time.

I am not that familiar with Barbarians and why they are broken but if they have Y throw X, they have X throw Y or A or Q.. Adjust to the party to keep it interesting..

I’m really confused by the force damage thing and wondering if I’ve missed something - some explainer for why they made this change. Why are a heck of a lot of creatures weapon attacks (eg a White Abishai’s longsword, a Steel Predator’s claws) now doing force damage?  “Force (damage) is pure magical energy focused into a damaging form. Most effects that deal force damage are spells, including magic missile and spiritual weapon.” as per the basic rules. 

Most Barbarians don't resist elemental damage anyway. The characters this force damage really hurts are like, tieflings and dragonborn, fiend warlocks, dragon sorcerers, anyone with Absorb Elements.

Edit: I'm referring to the spell attacks. The weapon attacks are just dumb.

I suspect there's a lot of force damage because it makes monster's threat fairly consistent, as force resistance is rare. Personally not how I'd do it, though I saw a fair amount of that in Fizban's as well. The real problem isn't that resistance exists, it's that monsters are tuned on the assumption that the PCs never do anything smart.

As far as monster design goes, I don't take offense at spellcasters losing spell slots, they're a big hassle to deal with, but they really need to make all monsters (spellcasters and otherwise) more interesting, most monsters in 5e are just bags of hit points that do damage and nothing else.

Im on board with the change to spellcasters. I dont think NPC spellcasters should be using the same mechanics as a PC spellcaster. A player will have time between each of their turns to think about which of their dozen spells to use and what level to cast it at. On the other hand, Its tedious as a DM if I am running a spellcaster NPC with some minions to try and juggle taking the time to decide which spell will be the best use of their next turn while also deciding what level to cast it at while also trying to keep the rest of the combat encounter moving with the PCs and minion NPCs.

Generally, the spellcasters in my game dont usually see enough time in combat to make use of their fully allotted spell slots to begin with, because the party I DM for is very quick to focus the spellcaster, so I do not think much is lost by trimming it down a bit. 

I generally like the changes. They highlight the most effective attack patterns for a monster or NPC, making the challenge presented by each more consistent across games. The change to force damage is a bit bizarre. It seems like they were added specifically for the purpose of removing the "Magic Weapons" trait entirely. Otherwise, I like having the spellcasting trait added to actions, and I appreciate the new focus on adding reactions and bonus actions.

Trimming down spellcasters was a good change IMO. If a given caster needs a different spell not listed outside of combat, then a DM should always have felt comfortable arbitrarily giving it to them.

Very Disappointed with the changes to pretty much everything. Removing spells and replacing them with "Big Number" attack is reductive and boring design decision. It makes spellcasters just ranged attackers instead of something that might actually make a difference on the field. So many classic creatures lost cool and interesting choices both in and out of combat.

The changes to some creatures getting force damage absolutely makes no sense and completely confounds me, it does nothing but gets around a players defenses and not in an interesting way. PC's didn't become more susceptible to damage the creature just does a whole different kind of damage then what would make sense from their attack.

I'm very concerned with the direction WOTC is taking with these changes. It's reductionist, it's lazy, and it doesn't make any sense.