The first thing that comes to mind is that they look much more unique from class to class compared to humanoids!
There are the fierce, claw-and-fang wielding monsters, as well as the cute looking kind.
They also have many different ways that they do things.
They have different ways of moving and attacking.
For example, Dragons look more dynamic, and Alraune (plant monsters) use vines to attack.
There are Many Other Types, as Well!
Unlike humanoids, monsters can't Lift characters, but they can Receive, which allows them to pass units even further when thrown!
Monsters pass units the same direction they are facing when Receiving!
Talking to Monsters!
Your conversations with monsters will change as your likeability with them changes.
Talk to your monster allies in the Overlord's Castle.
Turn the Overlord's Castle into a Monster Haven!
Monsters can use the Dark Assembly, too.
You can also make it so that you can walk around as a monster...
Maybe you can create so many monsters, you'll turn the Overlord's Castle into a monster haven!