ACKS' proficiency system provides a relatively low-overhead way to produce some mechanical variation between otherwise nearly-identical low-HD humanoids. Consider the following assignment of proficiencies to monsters:
Orcs: Weapon Focus, Berserkergang
Hobgoblins: Combat Reflexes, Fighting Style: Polearms
Tucker's Kobolds: Fighting Style: Missile Weapons, Skirmishing
Ratmen: Ambushing, Skirmishing
Lizardmen: Combat Trickery (Knock Down, Wrestle) (and then arm them with bolas, nets, and whips)
Gnolls: Fighting Style: Two-Handed, Precise Shooting?
So orcs hit unreliably really hard and don't retreat, hobgoblins are alert, organized, and quick, Tucker's Kobolds disengage and plink, ratmen hit-and-run, lizardmen disrupt the shield wall, and gnolls have heavy, slow melee supported by archers. Maybe add an extra * for XP purposes to some of these. It's no full charaterization of monster behavior, but it's sort of a set of mechanical hints towards behavioral characterization.
This is also a nice way to sort of experiment with the metagame, and give proficiencies that don't see much use by PCs some time in play. I wonder if there's a PC Proficiency / DM Proficiency dichotomy like there is with spells?