All right, should be cool!
Ruthless (hell) difficulty is removed - rebalanced the game across act 3
Merveil consistantly spawns exploding fire-monsters and throws them at you
Rename/recolour stash tabs
Changed death penalties: 0% Normal, 7.5% Cruel, 15% Merciless
New resistance penalties: 0% Normal, 20% Cruel, 60% Merciless
Legendary items have been rebalanced (mostly up) in power
Prismatic Ring - 8 (lowest roll) - 12 (highest roll) to all resistance
Added Quicksilver Flask - improved movement speed by 40% for 5 seconds (20 charges per use)
Bandit leaders now grant different stat bonuses on each difficulty level (Oak 45 from 40)
Cruel Bandits: 4% Attack, 4% Cast, 12% Physical Damage
Merciless Bandits: Maximum Power Charge, Maximum Frenzy Charge, Maximum Endurance Charge
21 maximum available passives through quests, compared to current state (16 points available)
Cruel is now actually hard. The game ramps up significantly near the end of act 3, where enemies can two-shot you.
The increase in level from map to map can be insane in merciless. The early zones can go from level 52 (Mud Flats) to level 58 (Tidal Island).
New passives: Five additional passives next to CI. Four of them being "6% increased ES" and one being "30% more ES"
New passives: 15% Elemental Resistances while holding a Shield
New passives: Keystone Candidate 126 - Life Regeneration applies to Energy Shield instead of Life
Haste buffed, gives movement speed (3% to ~5%)
Nerfs/Removal of stuff
Granite Flasks now grants 4k armor (from 10k)
Phase Run - Gone from the game, being completely changed
Lightning Arrow - Chance to shock is removed. Number of targets is now always 3
Dual totems nerfed: Ancestral Bond - Can summon up to 1 additional totem. You can't deal damage with your skills yourself
Chaos Inoculation - Maximum Life becomes 1, Chaos Damage no longer bypasses Energy Shield.
Clarity nerfed, 2.1 Mana Per Second @ level 1 (previously 2.5). 20% nerf
Discipline nerfed, 25% less Energy Shield granted - and 25% more mana reserved
Eldritch Battery indirect nerf: Discipline is worthless (costs more mana than what it gives back with EB).
Grace nerfed, 25% more mana reserved
Anger nerfed, 40 more mana reserved (essentially making it pointless, especially early-mid game)
Chain nerfed, 50% less damage (from 40%). Chains two times (from three)
Fork nerfed, 30% less damage (from 20%)