I'm not sure if I really needed to upgrade my Monster Manual. I mean, the old MM works just fine with the new rules. But I wanted to complete my set, so here we are.
So what's different? Hell if I know. I've cracked open the original 5e MM maybe five times. I mean, in the past ten years I've been a player more than a DM. When I did DM, half the sessions were online, where I tended to google the stat block I needed at the time. And the games I ran in person, I used a pre-made module with its own stat blocks.
I'll say this, I like the new look of the stat blocks. They're cleaner-looking, which is easier on the eyes. It's not a huge difference, but putting initiative in the top right, and putting boxes around the stats (now with their saves) is convenient. There's also a line for gear, for those DMs whose players loot everything they kill.
The monster's CR now also includes their PB, which I can only assume is the monster's peanut butter rating, something D&D has been sorely lacking. Those of us who use Reese's for monster minis have been demanding a peanut butter rating for years.
Edit: Okay, so I looked it up, and that's actually the monster's proficiency bonus. I guess that could be useful, but I'm a little disappointed about the peanut butter thing.
The monsters are organized a bit differently this time, which may or may not make things easier. For example, in the 2014 edition, the dragons were all lumped together. In 2025, black dragons are listed together with the B's, green dragons are listed with the G's, and so on.
However, the "index of stat blocks," which was in the back of the 2014 MM, is right up front as part of the table of contents page this time. It lists everything alphabetically, so "Adult Red Dragon" is listed in the A's while the "Young Red Dragon" is listed near the end, despite appearing together in the book itself.
This is good. I do remember a couple of times when I was looking up a monster, I found the index more useful than the table of contents. Putting the index up front makes it that much more intuitive.
One thing that's causing a bit of controversy is that orcs are no longer in the Monster Manual. This struck me as odd at first. I mean, yeah, sure, they're a player race now, but that doesn't mean you won't still fight orcs.
But it makes sense in context. Traditional orc stats aren't hugely different than those of a strong human. The Monster Manual still includes stats for humanoids, including bandits, berserkers, knights, warriors, toughs, cultists, mages, spies, nobles, and so on. Every one of those is designed so they can be any humanoid race.
Some monsters are gone, but some have been renamed or replaced with something very similar. Thugs are now toughs, quippers are now piranha, and lizardfolk now use the "scout" stat block. This book contains more monsters than the previous book, so overall it's a net gain.
There's a list in the back of the book that tells you what new monsters to use in place of monsters that vanished between 2014 and 2025. So if you want to build an orc warband, use berserkers instead of the orc grunts, use the "tough boss" instead of the chief, and use the "cultist fanatic" instead of the shaman. So there's no particular reason orcs needed to take up space in an already crammed book.
Bottom line: I can't say the 2025 Monster Manual is a necessity if you already have the 2014 book. There are improvements, but they're nothing groundbreaking. Still, if I had to choose between the two books, I'd pick the 2025 version.