In 5th edition, (especially late 5th) The metagame was all about how many
AV 10-12 tanks you could put on the table or destroy.
In this kind of Metagame, Coteaz and Henchmen were very powerful. For 62 points you would get a Razorback that was very good against other razorbacks or similar. For 135 pts, you get the Psy-dreads who were great against this type of list as well.
This style of play took a lot of hits with 6th edition. Vehicles generally die faster and you need to get out of them in order to score. You 3 man squads have to expose themselves to return fire in oder to take an objective, instead of having an
AV 11 scoring box.
Having to get out of a transport to score on an objective has moved people away from the
MSU style lists to larger and more durable infantry units. More people are running lists on foot making your high
STR firepower less useful.
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DCA were very strong in 5th edition as well. With INT6, good
WS and attacks, and their weapons ignored all armor saves, a
DCA unit would wipe anything off of the table. You could multiple assault several units and be confident that they would all die. (I would run some crusaders with them to take fire in the shooting phase, but to each his own.)
Might of Titan was very strong in 5th as well. A lot of people were running a lot of tanks that would clog up the board. You could often get a multiple assault that would kill maybe 3-5 tanks, which would kinda win you the game against certain lists.
In 6th edition, this style has taken a big hit in power.
DCA were the kings of multiple assaults as they were insane damage at High INT. Multiple assaults were nerfed hard in 6th edition due to Overwatch from multiple units, and the loss of the extra attack.
DCA units now MUST include some number of crusaders to take overwatch hits, usually lowering their damage somewhat. Also
DCA must now choose between hitting at INT 6 (power sword/mace) or ignoring a 2+ save (power Ax at INT 1).
Might of Titan is no longer wanted to assault tanks. You WANT to only glance the tank to death with S4 so it is wrecked instead of exploding. Shrouding no longer combos with the Storm Raven to give you 3+ cover saves. The librarian has fallen out of favor quite a bit.
Lastly, there are some units in 6th edition that you can basically never charge into due to the strength of their overwatch shooting. (Ork Burnas, Daemon Flamers, or any unit with a large number of flamers, or even just 30 shoota Orks).
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Strike Squads have become a lot more popular in
GK lists. Simply putting more durable bodies on the table in order to take objectives is popular. S5 storm bolters are now a legitimate threat to tanks. They can glance down
AV 11 which is all of the Rhinos and Razorbacks, plus many of the flyers as well.
Inquisitors are now very very strong. The divination signature spell is one of the best buffs in the game, and
GK units are well equipped to use them.
overall
GK are a bit weak to flyers. The Raven is decent against other flyers but it is pricey. The Helldrake is very very good against
GK (all our best units have a 3+ save), and you have no good answer for it.
In general, most of the competitive players have moved away from
GK. They are still decent, but they are not the sweeping powerhouse that they were in late 5th edition. One of the more common netlists at the moment is the
CSM list with several helldrakes in it, which is hard for
GK to beat. This gives you common bad matchup that will keep you from consistently hitting the top tables. This means there is not a "power net-list" running around that all
GK players are using.
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TL
R
5th edition
GK was all about taking a ton of cheap henchmen razorbacks and then filling the rest of your points with whatever you wanted.
6th edition
GK is more about 10 man strike squads led by inquisitors stomping around.
NOTE: all of this is based on the metagame you see expressed in forums and online. Each area has its own group of players with their own lists. If you are still seeing the spammy tank battles of 5th edition, then go with a list that is good against that. If you are seeing a TON of Flyer heavy lists, go for something that is good against that.