Overview and Basics

Core Overhauls

Skyrim Unification Project uses a custom encounter zone and level scaling mod to delevel the world, instituting minimum and maximum level caps for every dungeon.
A custom leveled list system (similar but not derived from OWL) balances loot distribution and prevents high level gear from appearing outside of the strongest enemies.
Sacrilege and Growl overhaul the Vampire and Werewolf systems.
Dragon War
Lawless
Reliquary of Myth
Imperious and Andromeda
Wintersun

High level items have been hand placed at specific spots in the world, some more challenging to obtain than others.
Smelting or breaking down equipment requires the smithing perks of the relevant materials.
All spell books can be sold and purchased regardless of your level in that school of magic. Spellbook vendors will have random selections, instead of every spell you can actually cast.
Merchants will sell more of the types of ingredients that can be gathered in their region than ones from far away.