HPL2 vs HPL1 vs other indie engines

[Hannibal]

The Frictional team says they wrote HPL2 from scratch. I'm curious in what ways it's an improvement upon its predecessor, and how it compares to other modern indie game engines (C4, Torque, Unity) that they chose not to use, other than 'it's written specifically for this sort of game', if anyone has thoughts.

I noticed there were no true outside areas in Amnesia, which makes me guess it doesn't support heightmaps, voxel-based terrain, or skyboxes. It does have sweet support for dynamic shadows.