New Main Stat: Composure (Com)
Composure is made by adding your Wisdom score and half of your Intelligence and Charisma scores, then dividing by two. It's basically Constitution for Mental Health.

Rest Changes
Short rests are limited to two per long rest, but do not have to be taken together, and simply require five or more minutes of rest or light activity.

Long rests do not restore all HP-instead, they restore 1 Hit Point per level, and the usual half of your hit dice.

New Derived Stat: Mental Health (MH)
Mental Health is how stable and sane your character is. 0 or above, you are fine (though the lower you get, the more panicky and unstable your character should act). At -1 to -10, you have reduced resting. For each point in the negatives you are, you have a 10% reduction in rest benefits, rounded down. For spell slots, HP, and similar, you multiply the amount regained by the percentages, rounding down. For more limited things like Action Surge, you roll over the percentage to regain the feature.

Percentage Based Negative MH Things
HP
Spell Slots
Rages
Bardic Inspiration
Battle Master Dice
Ki
Lay On Hands
Divine Sense
Sorcery Points
Arcane Recovery

All other class features that are not at-will must roll over the percentage to be regained on a rest.

Examples
A 2nd level Barbarian is at -4 MH. He has a 40% reduction in resting bonuses, so regains 1 rage (.6*2=1.2, round down), no hit dice (.6*1=.6, round down), and 1 hit point (.6*2=1.2, round down) each long rest.
A 5th level Champion Fighter is also at -4 MH. She has a 40% reduction in resting bonuses, so regains 3 hit points (.6*5=3), 1 hit die (.6*2.5=1.5, round down), and has a 60% chance of regaining her Action Surge (41+ on a d100).
A 14th level Wizard is at -8 MH. They have an 80% reduction in resting bonuses, so regain 2 hit points (.2*14=2.8, round down), 1 hit die (.2*7=1.4, round down), 1 spell slot worth of Arcane Recovery (.2*7=1.4, round down), and no spells (.2*4=.8, round down, and every other spell level has less).

If your MH is past -10, you start losing HP and other resources in a similar manner. For each point lower than -10 you are, you lose 10% of the normal HP you would regain, and 10% of the percentage based features, rounded down. You have the same chance of losing non-percentage based features.

Examples
The same 2nd level Barbarian is at -14 MH. He loses 40% of what he would normally gain, so loses no rages (.4*2=.8, round down), no hit dice (.4*1=/4, round down), and no hit points (.4*2=.8, round down).
The same 5th level Champion Fighter is also at -14 MH now. She loses 40% of what she would normally gain, so loses 2 hit point (.4*5=2), 1 hit die (.4*2.5=1), and has a 40% chance of losing her Action Surge (40- on a d100).
The same 14th level Wizard is at -32 MH. They lose 220% of what they'd normally gain, so loses 30 HP (2.2*14=30.8, round down), all their hit dice (2.2*7=15.4, round down), all their Arcane Recovery (2.2*7=15.4, round down), and all their spells.

As you can see, it's important to keep your Mental Health high. That is not always possible, however. The most common cause of losing MH is simple darkness. For each minute spent in the dark in a dangerous place, you lose one MH if you fail a DC 5 Com save. The DC increases by 1 each minute. Many monsters are horrific enough in nature to force a similar save as well.

Classes, regardless of type, gain 1d10 MH per level.

MH can only be restored in apparently safe places-resting out in the wild or in a dungeon will NOT restore it.

No spell or other effect can restore MH-it can only return, at the rate of 1 MH per level per long rest, when resting in safety and peace.

Upon leveling, you gain 1d10 MH, but do NOT restore your MH to max (if your DM levels you up mid-dungeon).

Reincarnation And Soulbinding
Darkest Dungeon allows you to reincarnate when the party is wiped (and it will be wiped) or when a character dies. The new character starts at level 1, with only Soulbound equipment, and has point buy equal to 26+the dead character's level.

Point buy is modified as follows:

6 -2
7 -1
8 0
9 1
10 2
11 3
12 4
13 5
14 7
15 9
16 12
17 15
18 19

Soulbound equipment is gear that has been imbued with the ability to return to your descendants or successors. To Soulbind a piece of gear, you must expend 25 MH during a long rest, as the process takes a while and is mentally taxing. Magic items can take more. Generally, each level of rarity above Common increases the MH cost by 5. In addition, you cannot reduce your MH below 0 by doing this.

You start with your basic equipment for your class-all Soulbound. If, however, you reincarnate into a new class, your equipment does not change.

Race Changes
Dwarves, Elves, Gnomes, Half-Elves, and Half-Orcs no longer have Darkvision. They instead have Low Light Vision.

Tieflings retain their Darkvision-however, their spectral whisperings and visions they see when forced to use it cause them to suffer the same MH loss as if they were in darkness when using their Darkvision.