EDIT
I just found out about a way to shorten consecutive riposte recoveries, which on paper should still only solve a minority of the problems mentioned below while adding new ones, but I don’t like talking about something I have no experience with, so I decided to shut up for a while and see how this discovery works out in practice over the coming days. 
I really want to like this sword, it fits the exclusive shield combo, it’s as if he just puts the shield away, it is faction-specific and has interesting mechanics and beautiful animations.
However, the more I use it, the more I hate it. The design just feels so bad as if nobody has ever tested it. I am going to list the problems I have experienced and a few ideas that might fix these. I have been playing on both legend and cata but I feel its problems are much more pronounced on cata than on legend.
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Slow attacks for a 1h weapon.
This means you often cannot just interrupt sudden attackers on reaction and have to combine it with evasive footwork or a block. Both things that can’t always be used. -
Very slow recoveries into block. Even after swings you often have to dodge to avoid getting hit because of how long it takes to recover your swing into a block, with the exception of light1 which can be cut short if timed right.
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Poor cleave, at least on cata, meaning the amount of enemies swinging at you with impunity while you are also swinging at them but with your blade getting stuck on someone else is even greater and makes the slow block recovery even more disastrous.
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Useless riposte feature.
This mechanic is SO slow to activate (even later than the shields actually showing up) that you can not use it to block on reaction and often just letting a quick heavy fly to interrupt/kill instead of block will be just as fast. I need to do some more testing with the timed block window for the power buff, but if it starts together with the parry time window, it is practically over by the time riposte activates. Needless to say parry is so short that it only works reactively, not proactively, whereas riposte only works proactively.
Slowing down your movement speed to almost 0 also makes no sense as, on one hand you need to move into combat as you activate it, but on the other hand you won’t be going anywhere so you need to activate it when already a step away from striking distance, while doing so will get you hit since half the time strikes will come in faster than your riposte activates.
What’s more, it can easily be interrupted and delayed so that although it seemed like you started holding the attack button early enough, you are going to get hit with a flurry of attacks within a single second until you let go and block/dodge instead. -
Not enough stamina!
It has 1 less shield, that is 2 less stamina, than the regular 1h sword, making the whole riposte idea even more nonsensical unless you are using the already very difficult to use parry, which as noted above does not work with riposte because of offset time windows.
Now obviously to “fix” the sword and make it more competitive, and especially to restore the synergy with GK’s kit FS originally had planned I assume (again, did nobody test this?!), I would do all or at least some of the following.
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MAYBE speed up at some of the attacks a little bit, like the light2, unless it would boost dps too much, I don’t want another OP weapon.
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DEFINITELY bring block recovery in line with the other 1h weapons, or at least the 2h weapons like the exe. Because being able to react to things is 50% of combat.
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MAYBE increase cleave, it is a 1 1/2 hand sword after all, meaning it has the most control of all weapons when wielded with 2 hands and should not get stuck easily.
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DEFINITELY fix riposte!
It needs to activate at least as fast as the regular block in order to be a worthwhile alternative since block comes with the edge of a a push and push attack combo, so riposte will need its own edge. -
MAYBE improve stamina, stamina recovery or block cost, but DEFINITELY make it sync up properly with parry so that people who want to focus more on defense than on offense can do that with parry, as it works on every other weapon.
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HOW ABOUT additional synergy?
Currently, the crit talent feels like a gimmick. It is a lot of fun and impressive damage whenever it hits a bigger enemy that wouldn’t just die from 1 or 2 non-crits anyway, but it happens so rarely because of the really low max crit chance of 15% you can get on the GK.
I am currently between 50 and 60 wins with GK and about as many failed cata runs the challenge meter obviously doesn’t count, and neither did I, but there never seemed to be a stark difference in terms of damage numbers between crit build and power build, which suggests to me the crit talent provides roughly the same overall damage as the power talent but much less reliably and with the added requirement of slotting crit on trinket and weapon (not a big deal for trinket as on cata you cat forego CR in favor of stamina recovery), meaning you have to drop a defensive stat like BCR or stamina from your weapon, which the GK desperately needs because he has very low stamina management and no real damage reduction either.
On top of that, power comes with other benefits like increased cleave and stagger, which even work great with your shield.
So how about giving the Bretonnian longsword and maybe even the sword and shield +5% crit chance across the board to actually make the unique and fun crit talent a little more reliable and competitive?