So since you're asking about it from a GM perspective, I'm gonna say that my initial impression is "meh." They addressed some issues with spells! But not others. They created some issues where there weren't any before, but they did make some things that'd confuse players easier to understand- even if I don't like how they did some of it. I see a lot of questions flying around and a lot of the answers are "hopefully it'll be in the DMG," but ... we don't have the DMG, and we have to hope that they're addressed in there.
I'm gonna be negatively biased, because they buffed player characters.. and I don't think that was necessary. I understand that if they make PCs weaker no players would buy the books. But right now, we don't have a new MM to see if the monsters are going to be stronger to offset the more powerful PCs... or is it just supposed to be a feel-good stompfest? I think there's a misalignment in my expectations of how strong, for example, an ogre should be in relation to a PC, versus how strong they actually are. And that's definitely more of a personal preference thing formed by previous editions and other systems, but it does affect my opinion of the book as a GM.
And if monsters ARE made stronger to offset the more powerful PCs... now we're just dealing with higher numbers vs higher numbers? Blech.
So I guess it depends on what you value as a GM. A lot of people seem very focused on the idea of making the game as approachable as possible so new players can join the hobby... I think that's super great for WotC's bottom line.. but for a GM that has already cultivated a group of great players? It's really not a big concern.
I love the hobby and think that more folks getting into TTRPGs is a positive, but while the format is better for teaching players how to play the game.. how is giving characters more and more things to do each turn in aid of helping new players learn the game? It seems like it'll slow play, and make the game less fun.. even as each character has more powerful/cool stuff to do in that turn.