Oh no, yet another GK foreboding thread! So please let's try to keep it constructive.
As we've seen, the new wargear, powers and rules brought by the codex affect different armies in many different ways. Some will have to re-think their lists and strategy a lot. Some will only need to tweak a thing or two and be mostly alright. Some may even be cracking their knuckles and welcoming the new GKs as easy prey.
So which is which? Here's my 2 spent bolter casings, and plase submit yours as well.
-Daemons: Quite boned, though we're yet to see how much. The psy-negated inv saves are gone, which is a bit reassuring, but between the wonky DS-interfering units, odd force halbers here and there making you afraid to charge your GD, and purifiers scaring your hordes, as well as the army-wide Favored Enemy (which is quite fully, come on), there's a lot to digest.
-Tyranids: Not quite boned, but shaken. Nidzilla is a bit shaky faced with mass Force Weapons (though they have Shadow in the Warp to even things out), and a shooty GK list can pump out prodigious amount of S5-7 shots that will make every save on your heavy hitters count, and you'll me making lots of them. Again, purifiers will make hordes pick their targets carefully.
-Orks: The Green Menace may be one of the most affected armies that few people actually realize. Again, purifiers will bonk hordes. and you can do so much cheap stuff with them, like combat-squadding them for a double dose of 4+ unit-wide wounding on everyone. One of their power units, nobs with varying wargear, loses it entirely raison d'etre against the force weapon smogarsboard. Lootas can do a good job at knocking the Stormraven and other non-LR transports down, as well as forcing saves on the Dreadknight, but any interceptors will beeline for them. and they have zero psychic defense.
-Chaos Space Marines: Oddly less affected than it should be, really. Few people use summoned daemons competitively, and even the Daemon Prince doesn't have the explicit Daemon rule, weridly enough, though making him a psyker makes him vulnerable again. Maybe some people might swap their warptime princes for a Khorne mark? Otherwise, they best avoid charging zerkers at units with halberds. The only 2-wound units that are under extra danger from Force Weapons are Oblits, who are not complex anyway and if they are being charged, they are likely out of the game already. Melta/Plasma weapon combo on 10-man csm squads may become popular to help deal with Volton-knights. Deepstriking termicide may become very tricky, though. There goe the last use for chaos termies? And vindicators become yet more valuable to deal with paladins if the enemy wants to splurge!
-Vanilla SM: Average. Given the fact that waiting until you have melta range means bringing the heaviest firepower of the GKs to your face, melta and multi-melta in tactical squads may be replaced with more lascannons, who are also better for swatting Stormravens. Upgrading librarians into firing 2 powers a turn might become more popular to do some Force Weapon havoc on ICs with Ini 10. Smite will also be a good termi-troop hunter, even with the Ld penalty. Assault termies can make short work of Dreadknights and dreads. But Vindicare shooting the rosarious off Captains and other HQs may become a concern!
-Tau. Low impact: Interceptors will be scary against Broadsides, as well as shunting and other tricks will truble Crysis suits doing the jump-shoot-jump hiding trick; people may shift more toward Hammerheads upgraded to fire as Fast vehicles to drop the GK mech and then pump S6 plates on the move. However, Aun'va and ethereals in general curb-stomp a surprising number of the GK's best heavy hitters.
Kidding. April 1st -is- coming.
-Blood angels: Little to none. Fast Razorback spam works very weel agains the mid-ranged GKs, and if the tries shunting-intecepting a wily BA player can make him spread his forced wide and unable to support each other. Since GK don't seem too keen on power fists, though, Dreads with blood claws may be quite amazing to deal with squads. laughing as force weapons do nothing against its hull as they stomp silver heads. Getting them to assault range while avoiding psycannon fire may be tricky, however. Also, a new challenger arrives! Can Draigo beat Mephiston? Can the man who gave Mortarion an internal tattoo, stomped on Nurgle's prized daisies and foursome'd Lelith Hesperax into a sated kitten all by himself consistently outperform everyone's favorite dwarf Swarmlord?
That's all I got in me right now.