When I first played against her, I thought it'd block the first enemy auto attack or ability, similar to Sivir or Nocturne's spell shields but blocking an auto attack as well.
So why exactly does she get a .75 second 
ult/
black shield on a basic ability?
Kratos2634 votesIf you want to have a good idea how to beat her, play her a bit. You'll be able to learn what to do and what not to do.
LankPants4 votesWhen I first played against her, I thought it'd block the first enemy auto attack or ability, similar to Sivir or Nocturne's spell shields but blocking an auto attack as well.
Unlike those abilities Fiora can't move or a attack while she's using Riposte.
If she was disabled for 0.75 sec and only blocked the first thing that hit her using the ability would almost always be a bad idea. If she wasn't disabled for the full duration it would have the potential to be a lot stronger in certain situations anyway.
DropFill3 votesBecause it lasts only 0.75s, she needs to stay still, and the CD is hella high. Her problem is the maximum health true damage.
BiBoX21 votesIt's a better SpellShield, blocking all the burst/damage and even the (empowered) AA, including the on-hit/on-spell effects.
It's better than Yasuo Wind wall when vs Lux's ult or Nautilus's Ult.
It's better than Fizz's pole vs Darius ult or Garen's ult.
Even "unstoppable" ults like Vi's , Sion's , Malphite's can be void with her non-ult parry :) .
So this skill only has compensated for her "untargetability" of old ult, lower CD than Yi's Q, and even has CC retaliation!
DropFill1 votesIt's a better SpellShield, blocking all the burst/damage and even the (empowered) AA, including the on-hit/on-spell effects.
It's better than Yasuo Wind wall when vs Lux's ult or Nautilus's Ult.
It's better than Fizz's pole vs Darius ult or Garen's ult.
Even "unstoppable" ults like Vi's , Sion's , Malphite's can be void with her non-ult parry :) .
So this skill only has compensated for her "untargetability" of old ult, lower CD than Yi's Q, and even has CC retaliation!
Yi Q gets reduced by AA'ing "Better than fizz pole" , they serve totally different purposes. Stop comparing her W with untargetable abilities. You compared her W to yasuo windwall lol, how pathetic.
Ralanr3 votesI think she can't Parry Mordekaiser's third Q. I read that in the wiki but haven't seen it.
Kratos2632 votesI think she can't Parry Mordekaiser's third Q. I read that in the wiki but haven't seen it.
Nah she can, I've tried it. It must be really depressing for the morde too, because you still hear the satisfying "Ting", but there's no damage.
Ralanr2 votesWell that's depressing since Pantheon can't block it.
At least he ruins her lane.
linkmastr0012 votesIt's everything from the direction she faces it.
DorkunedAuras1 votesIt's everything, everywhere, not just directional. that would give counterplay...
Kensue1 votes[deleted]
BiBoX21 votesYes, she parry all the ability/AA regardless of the direction. But the retaliation is directly against the offended direction (if not directed, then toward the hostile champion).
A good example is in game 2 SKT vs KOO (Worlds 2015 Final), Bengi's Jarvan tried to knock-up Smeb's Fiora from behind, but Fiora parried it and almost escaped.
Kratos2632 votesLook at it this way, once she presses W, she becomes invulnerable for 0.75s and MIGHT stun you if you used hard cc and you're in her aoe. It also has a pretty long cooldown as mentionned before. However that's not what makes her broken, since there are other champs who can become untargetable/invulnerable like fizz, yi, vlad, and I beleive they all have a low cooldown on it.
(Also black shield IS a basic ability, so I don't see why no one else may have it)
At least she can't move during it unlike other champs.
Sugar Momma2 votesFizz yi and vlad all do NOT become invulnerable during their untargetability. Common misconception. They can still die to DoT's like 
Kratos2632 votes[{quoted}](name=Sugar Momma,realm=NA,application-id=3ErqAdtq,discussion-id=hsEN7zaF,comment-id=00030000,timestamp=2015-11-01T17:19:04.331+0000)
Fizz yi and vlad all do NOT become invulnerable during their untargetability. Common misconception. They can still die to DoT's like
yeah thats why I said untargetable/invulnerable. Here's an exemple of invulnerable :nocturne and sivir. Now, fiora's parry is fairly easy to bait, and once she does, you have 19second to punish her. Also, as I said, she's the only one who stuns herself while doing it, which means a badly timed riposte is much more punishing to her than it is to fizz and sivir for exemple. It's not overpowered like most people seem to think.
BakedLotion2 votesyeah thats why I said untargetable/invulnerable. Here's an exemple of invulnerable :nocturne and sivir. Now, fiora's parry is fairly easy to bait, and once she does, you have 19second to punish her. Also, as I said, she's the only one who stuns herself while doing it, which means a badly timed riposte is much more punishing to her than it is to fizz and sivir for exemple. It's not overpowered like most people seem to think.
Actually spell shields arent invulnerable since they dont block anything but one spell. They can still die to DoT lol. They literally only block one spell. That's no where near invulnerable. Spell shields are spell shields.
Vistha Kai3 votesActually spell shields block every spell that hits at the same time.
BakedLotion1 votes[{quoted}](name=Vistha Kai,realm=EUNE,application-id=3ErqAdtq,discussion-id=hsEN7zaF,comment-id=00030000000000000000,timestamp=2015-11-01T19:28:34.279+0000)
Actually spell shields block every spell that hits at the same time.
still not invulnerability since it doesn't make you take zero damage from anything used before it
Flyer Beast1 votesDoes 
's W block basic attacks?
I mean I heard about she can parry anything but even mere basic attacks that aren't a spell???
Sugar Momma1 votesYeah, it's pretty BS
Flyer Beast1 votesOh Shoot. That's extreme
Bronzoid1 votesIt used to be complete shit, so they made it completely broken.
Balance isn't something Riot is capable of.
AzureInTheDark1 votesShe can't riposte tower shots.
3TWarrior1 votesbut vlad and fizz can XD
3TWarrior1 votesdon't forget that it's also an offensive move that increases in power when it parries something...so basically a Jax's E without the stun...oh lol, it can even stun if it deflects hard crowd control