A strict enemy level system that correlates with the PC.
I just started a new game on nightmare for the Ego Draconis part (DKS). I explored the Orobas Fjords too early. When I got back to first meeting Lovis, I was already level 21. There were odd things I noticed. Magic users seem to run out of "magic" and there are points when they would just run away (some imps I came across on the Fjords). This happened when I was power leveling there.
Are the enemies strictly customized? I'm not sure but as a level 21 going back to Broken Valley with assortments of level 8 - 14, the level ranking seems inconsistent at points. One may argue that freeform character development may have something to do with this but games like Neverwinter Nights has a reliable Challenge Rating which remains consistent all throughout.
