The Tear of the Land

According to one of the developers the only thing that changes with difficulty is monster behaviour in combat,aka the higher the difficulty the stronger or smarter the enemies get.

Quote:

We have three difficulty settings that scale monster behavior in combat. On top of that there's old school mode which disables the auto map. We are also considering adding the permadeath option in a game update at some point but completing the game with this option would be very, very, VERY difficult I believe.

Source: Legend of Grimrock's official forum

Reach weapons don't have a seperate skill, but they have a property that makes them somewhat usefull never the less:
They can be used from the back row.