First off, the Crimson Slaughter army is a Chaos Space Marine army for all intents, and follows that book more or less by the letter. The exception is that all models in the army cause Fear and that Possessed can be taken as troops.
Possessed also replace their normal Vessels of Chaos rule with a new table rolled on at the beginning of each Chaos turn. On a D3 of 1, the unit and transport they are in get Shrouded, on a 2 they become Beasts, and on a 3 they get a 3+ Invul and Rending USR. All of those are pretty good, but so unpredictable and different it's hard to create a plan around them.
Another Crimson Slaughter introduction is that one unit of Chosen can be upgraded to Draznicht's Ravagers for the cost of a meltagun, which gives the Champion preferred enemy.
The only units in the Crimson Slaughter detachment that can take the Veterans of the Long War upgrade are Plague Marines, Bezerkers, and Noise Marines.
The new warlord table gives the following bonuses: hatred, with the bonus that the warlord and unit always re-roll misses in combat against Dark Angels; all enemy units within 12" of the warlord suffer -1 to their leadership and -2 to leadership for fear tests; furious charge, though he cannot shoot if an enemy is within 12" at the beginning of the shooting phase and must instead attempt to charge; Crusader USR for Warlord and unit; Warlord inflicts D6 S3 hits at I10 on each enemy model in base contact; and Shrouded USR.
There are several new relics, which replace those from the CSM codex. The first is the Crozius of the Dark Covenant, at the same cost as a Spawn, which gives +2S, AP4, and confers the Zealot USR on the bearer and friendly units within 6".
The Blade of the Relentless comes in at the same cost and has the base stats of a Power Sword, but gets bonuses depending on how many enemy models killed in combat by the bearer. At 1 or more, he gains +1S, the sword becomes AP2 at 3+ kills, an additional point of strength at 5+, and causes instant death at 10+.
Next is the Slaughterer's Horns, which come in at the cost of a plasma gun and confer the Furious Charge, Hammer of Wrath, and Rage special rules.
The Balestar of Mannon might be my favorite, which can only be taken by a sorcerer, at just over the cost of a plague marine, and allows the sorcerer to generate from Divination and re-roll failed psychic tests. However, he and his unit gain no modifiers to Deny the Witch rolls (so always deny on a 6, only).
Next up is the Daemonheart at the cost of a spawn, which cannot be taken by a Daemon Prince and gives a 2+ Save and the It Will Not Die USR.
Finally there is the Prophet of Voices, which confers the Daemon, Fearless, and Fleet USRs on the bearer for the cost of a Spawn. The bearer also rolls on the same chart as possessed and can only join units of possessed.
And rules wise, that is it. There are also a few Cities of Death Stratagems and 9 different missions you can use to recreate battles in the Crimson Slaughter's fluff.