One of the reasons I expect Borderlands to be a lot of fun is the dynamic comments that give individual enemies personalities:
The amusing outbursts of enemies makes them more than just faceless meatbags. The Halo series has proven how effective this can be.
Giving enemies dynamic outbursts does two things. First, it adds a personal dimension to combat, making it feel like your actually fighting individuals and not the Borg. Second, it adds variation and replay value. It can make one enemy feel slightly different from another despite both using the same model and the same AI.
This isn't only possible with human enemies. Individual dogs of the same breed can have different barks. Individual skaags or other beasts could have different growls, screeches, etc. Voice variation with animals and monsters has an effect similar to that with humans.
Nor does personalizing individual enemies have to be done only through modeling, AI, and voicing. There's also animations. Slightly different hand motions, ways of walking, relaxed and defensive stances, and so on can make enemies which are otherwise the same feel like true individuals.
Every aspect of NPCs changes the experience.