Basic, Bookwide Stuff
So I am going to go over some bookwide changes to rules, marks, and things that will effect everyone. Starting with the marks or chaos.
Mark of Khorne - Gives Frenzy, the same as before. Good for making hitty units hittier. Hammers or shock units should probably take this mark or another one later down the list.
Mark of Tzeentch - Gives +1 to ward saves, and since ward saves are stackable with parry saves, all troops with shields get a nice 5+ ward in combat. Doesn't give bonuses to casting anymore though. However, they can reroll channeling rolls of one - giving them an additional whopping 2.8ish% chance of channeling.
Mark of Nurgle - Now is better. Does nothing at range, but enemy models are at a straight up -1 to hit in combat, not -1 WS. Combined with Miasma, some other spells and a warrior's base WS of 5 and you can have most things hitting them on a 6.
Mark of Slaanesh - Makes a unit immune to fear, terror, and panic. Got much better by virtue of will of chaos dissapearing and thins mark being half the price of the others. If you have a shock unit with enough power, consider giving them this mark so they make those important chargesat their real WS, and never run away when, say, a unit of hounds get destroyed.
One more thing about marks. They are now like they are in 40k - a character with a mark can only join a unit with that mark, or an unmarked unit or vice versa. Also, 2 characters with different marks can not join the same unmarked unit.