Difficulties no longer drastically change the damage multipliers and only change the damage of enemies
Fixed Problems with high damage multipliers/health on many enemies
All actions cost stamina. 70% or more stamina = damage bonus, 30% or less = damage reduction
Blocking with shields or weapons is much more efficient, heavy shield bash is now available
Heavy Armor is better but will slow the wearer and will increase mana cost for spells
Followers like Calia/Jespar are no longer invincible and will fall down if they would die
AI reacts better, attacks/blocks more frequently
Archers now use Arrows and will switch to melee weapons if needed
Certain creatures (like Bears and Trolls) will knockback the player with some attacks if the player isn't blocking
Undead have drastically increased armor and resistances. Added new silver weapons, silver arrows
Ghosts have drastically increased resistances. Bypass it with silver weapons or with a new consumable called "Ghost Curse"
More diversity between creatures. Some Examples:
Mud Golems (Troll Template) regenerate Health very slowly and have increased resistances
Steppe Crusher (Deer Template) are more aggressive and have a new attack pattern
Boneshredder (Vatyr Template) are slower and have increased armor, magic/poison resistances
Crystal Elementals (Frost Elemental Template) have new resistances, no longer deal frost damage in an AoE around them, removed frost FX
Spiders have different poisons (slow, paralyze or increased damage poisons) and big spiders will knock the player back
Giants, Bears, Spiders, Trolls, Mud Golems have new attack patterns
Starling Guardians use their crossbow, Starling Centurios use their steam breath
35 new Potions/Poisons and rebalanced the others (in most cases they are stronger now)
25 completely new Spells (many of them didn't even exist in Skyrim)
Manacost is drastically increased, Manacost reduction perks are now mandatory to properly cast spells
Completely rebalanced elemental spells. Some spells were just much better than others of the same rank.
Removed Secure Chests and many hints that had unimmersive or simply too much information
Stealth/Pickpocketing/Lockpicking rebalanced and overall made harder
Loot is much more diverse. Animals/Creatures will drop appropriate items
Many, many other changes. More informations are below and in the changelog