Disclaimer: I read the Monster Manual for 3.5 E but never played it so my basis for comparison is minor at best.
A LOT of monsters are in this manual, 315 pages of them plus a pair of appendixes for creatures and possible foes. Thus there is a variety available for a dungeon master to throw at their players. Be it straight-up and uncomplicated brawlers such as hill giants, the cunning and numerous smaller creatures like kobolds and goblins, the paranoia caused by the classic mimic or spell-casting creatures like hags, this book has it all.
Except for celestial creatures. Demons, devils, dragons and giants all get large sections for their sub-species and society but the upper planes get less attention. There are three varieties of angels listed and some others like the unicorn but those are all high Challenge Rating creatures; mid-game at the earliest, and there is little more lore for them than other creatures. I understand the reasoning for this, at least, I think I do.
Players are encouraged to fight creatures that are generally evil or threatening. These are feral monsters, devious demons, and rampaging orcs. The number of situations where they would fight angels or unicorns is far more limited. Including them as allies runs the risk of making the players irrelevant. So why waste time on them?
Being a guy who likes lore and world building, I find this disappointing. Although, there is enough to homebrew something, and that can be fun too.
That is also something fun with this book. It's not something that I can get from other books, which tell a story. This one gives the actors for such a story. After a certain creature's entry, it is fun to imagine a small little scenario featuring them which makes use of the lore: their habits and diets and such. Also, I like to consider some way to effectively use their stats and features against potential players.
This is another book with gorgeous artwork. Every monster gets their profile picture. The celestials look majestic, the fiends look dangerous and some of the aberrations are just creepy, like the gibbering mouther. This is great for the theater of the imagination.
Trickster Eric Novels gives "Monster Manual for Dungeons and Dragons 5th Edition" an A+