In general, I mean that the developers put in a lot of effort to make the creatures' animations and appearance as closely MATCH their parameters as possible.
For example, take the Plague Zombies. Looking at their bandaged legs, swollen arms, barely holding their weapons, and huge bodies, you understand that these guys aren't very mobile, they don't have a decent attack, and they should still be quite durable. And when you attack them, you hear a massive creaking sound of tearing seams. They're practically sacks of... stuff! And how bravely the Dwarven Warriors defend themselves! And you can analyze each creature individually in this way.
In my opinion, neither Heroes 3 nor Heroes 4 has such a well-developed and realistic animation as in Heroes 5.