The codex author is Matt Ward, who we all know. The following is a ton of information not really broken down. It is a copy paste from various forums, mostly "Bolter and Chainsword" and "Heresy Online"of Ciaphas Cain answering questions from the codex. These were compiled somewhat over at 40k online. Take all of it with a grain of salt please.
Dreadknight comes with (And this is how you know Ward did it) two Nemesis Doomfists as default.
-Grey Knight Strike Squad Options:
-For every five:
-All the Nemesis weapons (Any model!)
-Various point costs
-Justicar can MC any of his weapons
-Henchmen, upon further examination don't take up a FoC slot but you MUST have an Inquisitor to get a Warband (One warband for each Inquisitor you have). And there's no limitations, so yes you might see Daemonhost, an Arco, and a Mystic in one group.
-Assassins DO take up an Elite slot.
-Librarian can take all of the powers and can be upgraded to Mastery Level 3 allowing him to cast 3 per turn. Also, he comes default in Terminator Armour. AND can take all of the various Nemesis weapons.
-Thawn, no, but he is Mastery Level 2. Not sure why, as Hammerhands is the only power outside of I Shall Not Yield that he has.
-Valeria has a LOT of equipment. Here it is:
-She's an Ordo Xenos Inquisitor
-Stats: 4 4 3 3 3 4 3(5) 10 3+
-Power Armour; Frag, Krak, & Psyk-Out Nades; Laspistol
-Graviton Pistol: Range: x" Strength: x AP: 1 Type: Pistol, One Shot
-Jacked it from an Alien Weaponsmith in the Ultima Segmentum apparently.
-Runes of Destiny: ALL (Armour or invul) Successful saves against her shooting or CC attacks must be rerolled.
-Eldar Runes she's taken over her career, but she ain't got no wraithbone batteries to make em work at full power
-Dagger of Midnight: Grants +2A (Included in profile) that MUST be rolled with different colored dice (Or separate I suppose) because if she rolls doubles with them, the attacks hit her instead.
-Stole this from an Eldar Pirate's corpse. Apparently the dagger is "controlled by a rebellious and bloodthirsty intelligence". Please perform an "Oogedy boogedy boo" after reading that.
-Hyperstone Maze: Can be used once per game in lieu of her CC attacks. Must be directed towards a character or MC in B2B contact. Target must roll a d6 and roll equal to or below their remaining wounds or be "trapped in the maze forever" AKA removed from play.
-Multi-faceted emerald that leads to a spooky sub-dimension, apparently
-Forceshield: 4+ Invul save
-Hand-mounted forcefield thingy.
Also, randomly enough, they can take Ordo Xenos inquisitors. Weird.
-And yes, VenDreads are the only ones with Reinforced Aegis
Edit: Justicars are in. True Grit is out as far as RULE goes, but I think Stormbolters count as pistols for them anyway.
-Move Through Cover, Fleet, Infiltrate, Fearless, Uncanny Reflexes (4+ Invul)
-Deadshot: In Soviet Russia, Vindicare shoots you. Kidding, the Vindicare's player picks who he hits, not the opposing player.
-Exitus Pistol: x" S: X AP:1 Pistol, Sniper
-Exitus Rifle: xx" S:X AP: 1 Sniper, Heavy 1
-Rounds: Must Declare which round before rolling to hit
-Hellfire: 2+ to wound
-Shieldbreaker: Takes no damage, but PERMANENTLY removes Invul saves granted by items or wargeat
-Turbo-Penetrator: Inflicts 2 wounds on any non-vehicle model wounded. Has penetration of 4D6 against vehicles!
I Shall Not Yield! only works on Thawn, sorry.
As for the other assassins:
-Neuro Gauntlet: Counts as Lightning Claw
-Frenzon: Gain d6 attacks on the turn they charge instead of 1
-Executioner Pistol: xx" Sx APx Pistol, Poisoned (2+)
-C'Tan Phase Blade: Power Weapon. Instant Death against unsaved wounds regardless of Toughness
-Neural Shredder: Template Sx AP x Pistol Hits are resolved against Leadership not Toughness. Can't hurt vehicles.
-Polymorphine: When the assassin arrives from reserve, choose an enemy unit. Chosen unit immediately takes D6 Sx AP x hits. Then the assassin is placed anywhere within 3" using Deepstrike rules but does not scatter.
-Hit and Run & Stealth
-Psyk-Out Nades, Psyocculum (<- Not actually sure what that does)
-Animus Speculum: xx" S5 AP1 Assault 2. For every psyker within xx" of the assassin, add 2+ to the Speculum's assault value
-Etherium: Any unit wishing to shoot or use a psychic power on the Cullexus must pass an LD test on xd6 (Vehicles count as LD10). If the test is failed they can't target the assassin but can target others.
-Jaon on Dakka Dakka: Assassins can be taken in any of the 3 elite slots, but each is UNIQUE thus only one per TYPE of assassin.
Grandmaster: 6 6 4 4 3 5 3 10 2+
Brother Captain is the same minus a BS point.
Libbies, see GM stat line -1 WS -2 BS -1W -1I -1ABC and GM can take Psycannon, Incinerator, Psilencer, Servo Skulls (Up to 3), Blind Grenades, meltabombs, digi weapons, psybolt ammo, empyraen brain mines, psychostroke or rad grenades, and an Orbital Strike Relay. Oh and MC weapons. And all three are alll in Termi armour.
EDIT: Fluff questions! You're my hero. I'll answer the Dreadknight one for Brevity's sake, I'll answer the others later, if you guys come up with a list.
No one knows where they come from, be it Xenos or Dark Age of Technology stuff. The GKs basically refuse to say and tell everyone to F off who asks where they came from. They're basically used as Jes said on Warseer, to go toe to toe with big nasties like Daemon Princes and such. And apparently this thing is super hard to master so that's why we don't see more.
-Mordrak and Draigo do have The Grand Strategy
-Psycannons and Incinerators do not ignore Invuls as far as I know. Psybolt ammo seems to, though.
-No vehicles/walkers have access to Psycannons, Incinerators, and their ilk, outside of the Dreadknight which is a Monstrous Creature anyways.
-Dreadknight Options: (135 pt Base)
-Heavy Psycannon - 40
-Gatling Psilencer - 35
-Heavy Incinerator - 30
-Nemesis Daemonhammer - 10
-Nemesis Greatsword - 25
-Personal Teleporter - 75
-Also, Dreadknight Armour: 2+/4++
-Dreads(115 pts)/Ven Dreads (175 pts): Weapons are the same as they've ever been for a Space Marine Dreadnought. Fancy GK vehicle upgrades including Psybolt and Psyflame ammo.
There is a whole other series of posts, I will get these later.