The basic idea is....
Metal armor becomes more effective at protecting.
Conversely, all weapons are given higher damage values, with some damage types changed (in some mods, axes are made piercing, stabs with a sword are made cutting, etc.)
Bows are changed to cutting instead of piercing, so heavy armor is actually of some use, but because of the increased damage values, you won't wind up with a naked looter marching towards you with three arrows sticking out of him.
Horses are usually made faster and more maneuverable, with fewer hitpoints.