Caveat: I run the MM through DDB, so while I acknowledge some weird organization choices in the physical edition, which I also own for reading purposes, these aren't a factor in how I use the new MM.
The mosters are significantly improved in most cases. They are tougher, and because their damage is now usually spread among different damage types (e.g. slashing AND lightning damage, etc.) they are more challenging against things like a barbarian's rage (note: this was a de facto nerf to barbarians; they are now clearly the weakest of the four main marrtial classes, IMO - fighters, paladins, monks, and barbarians).
The text descriptions have good flavour without too much lore, letting DMs choose how the creature fits in their setting. For me, this is a very positive change.
There are more flavourful options for a lot of monsters, making the battles more tactical, except...
Spellcasters SUCK. This is a change that began with Monsters of the Multiverse: primary casters now just get a handful of spells instead of full lists, including creatures such as liches and archmages. I HATE this change! It makes them way more predictable in combat. I understand that it makes running them easier for novices, but most DMs know the rules and aren't bothered by having multiple spell options to choose from. Fortunately, DDB makes it easy for me to just use the legacy versions of spellcasters in most cases.
I also dislike that fantasy staples like elves and orcs aren't individually represented. Yes, I know that you can use statblocks like "guard" for any of them, but that takes away a lot of flavour. How hard would it have been to have one or two representatives of those classic archetypes, with a special ability or two to make them distinct? Orc warrior, Elf scout, etc. This is made more strange by the fact that some other classic species are represented this way.
Alignment is still used. I know it's a legacy thing and some folks insist on it, but I strongly dislike it and wish they'd at least stuck with the MotM convention of putting the word "typically" in front of it. And yes, I know this is mentioned in the forward but it still irks me.
