creature depth

In S.T.A.L.K.E.R., dogs act differently depending on how many fellow dogs are around. When alone, they run away. In large packs, they will attack. And when the player kills most of the dogs in a pack, the survivors lose courage and run for safety.

This sort of depth, simple as it is, adds considerably to the richness of a gameworld. It could potentially add strategic depth as well. Notice that it's not just behavior but also goal priority which changes in that dog example.

Connecting AI behavior to stuff like population and environment is something I'd like to see more of.