Things like rain going through overhangs, arrows perpetually sticking to the player, and those obscene ragdoll physics when a monster dies. There's no way it is a new engine built from the ground up. It feels more akin to Valve's Source engine in which they use modular upgrades on the original engine.
The ragdoll physics come from the Havok Physics engine.
The same peculiarities are seen in every game engine that utilizes Havok.
Havok svcks.