FAQ Day!

https://www.games-workshop.com/gws/content/article.jsp?categoryId=1000018&pIndex=1&aId=3400019&multiPageMode=true&start=2

They updated 99% of the 40K books, including the Necron Codex.

Necrons
1.  Taking two Courts means you can attach two Royal Court members to a unit.  (Huge Buff)
2.  Abyssal Staves calculate ID against Leadership... not toughness (aka they don't) (Nerf)
3.  Created Scarabs must be spawned in coherency with the unit's footprint before the turn started (Needed Nerf)
4.  Stormlord benefits from Chronometron (Buff)
5.  Stormlord does not get lightning attacks if you use Solar Pulse to see during your phase. (Nerf)
6.  Whipcoils got hit with the Lash Whip nerf, so modifiers such as FC or Halbreds apply after you get reduced to I1.  (Nerf)
7.  Trazyn does not replace a model until AFTER he has failed his Everliving Roll (Huge buff for him)
8.  Entropic Strike works before rolling to penetrate and is distributed evenly across squadrons.


Grey Knights
Grey Knights got a significant buff from the Whipcoil/Lashwhip ruling as now Halbreds allow you to strike at I3. 

Space Marines
Space Marines (and all variants) got a needed clarification about how Combat Squads works with reserves.  Now when the unit arrives from reserve, you can then choose to Combat Squad.  Not a huge deal, but very nice anyway. 

Nids
Unfortunately, GW didn't go back and retroactively remove their previous fails:

1.  Units with built in cover systems such as smoke or disruption pods do NOT get cover saves against Hive Guard. (Huge buff)
2.  A Tyrant and Guard only count as one KP
3.  Venomthropes do NOT cause your unit to strike at I1 if you do not have grenades (WTF)
4.  Shadows now affects units in vehicles (Huge, needed buff!)

CSM
Poor CSM got hit with another nerf... Lash now has to roll to hit.  However, Dreads only Frenzy with 45 degrees.

Overall, I am very, very happy with the Necron and Space Marine FAQs.  They cleared up almost all of the major issues in the Necron book and cleared up a nagging issue with Combat Squads.  I am not sure how I feel about the Nid one yet, though.  I think they provided some nice improvements, but that whole thing is still a huge mess.