I can't remember what game it's from (Lineage II, maybe?), but I remember reading that for their NPC/PC collision detection, if you ran at them for 5 seconds, you'd run through them. I think this would add that collision detection depth without giving fodder to griefers. Of course, I think it would be fun if they added something similar to Assassin Creed's 'Gentle Push' motion, where it nudges the NPC for a second so you could get past before resetting their position.