As a DM, I really like that the rules are more concise, for players' sake. I think the Rules Glossary is a bit of a missed opportunity tho. It's not just a place to find quick definitions of things but it makes the core rules spread between the front and back sections of the book, so you'll be flipping between those regularly when learning the rules. I'm sure in-play the glossary becomes more useful for in-the-moment referencing, tho
I like the new emphasis on feats, to make characters a little more diverse mechanically and to rebalance the out-of-whack PHB feats. The layout and art are better across the board compared to the 2014 PHB. I love that tools and adventuring gear all have rules and are actually useful now. I love that sorcerers and warlocks get more spells now, though I'm not sure wizards needed to have even more lol
I don't like how restrictive backgrounds (and therefor your origin feat and proficiencies) are. It's the thing I immediately threw out, letting people customize them and swap each part. I hate that a criminal rogue has two sources for thieves' tools proficiency and no ability RAW to swap one of them. I hate that each of the magic initiate feat options are restricted to one background each. I hate that class/background combinations are unnecessarily restricted--an acolyte paladin is straight up worse than a farmer paladin. A soldier can't start with tavern brawler. And there's little space to argue that these restrictions are part of some wider game balance or even to keep characters in some limited story design space, any scrutiny will see the backgrounds, their associated ability scores, and their associated feat are design decisions based on vibes alone. Yes it's just a +1 difference in your main stat, but we already knew from 2014 species design that restricted ability score increases restricts the character concepts you actually end up seeing at the table.
I really don't like that they barely did anything with the ranger class changes, it's definitely an afterthought despite how low they said the 2014 one was rating. Two-weapon fighting is still an absolute mess, just a different one than the 2014 PHB had (TWF builds have to base their play on a weapon property, a weapon mastery, a fighting style, and an obscurely-worded general feat), and statistically it still struggles to be worth the investment vs. just taking Great Weapon Master and a weapon with Cleave. And god do I hate that find familiar still has them acting on their own initiative when they already fixed all the other action economy annoyances with summons elsewhere.