HOW IS RIVEN OP?
She isn't OP, but her perception of being OP is due to several factors:
1) She's really good at killing scrublords and noobs.
2) She tends to win "harder rather than smarter" contests and is mostly delt with by "smarter rather than harder" tactics. Riven excels by dealing with situations through reflexive action rather than tactical or strategic plays.
It's been proven time and again a vocal community's strong point is not smarts.
3) Everything she does is super-obvious and is met with immediate validation. She wins the 5v5 game of LoL by killing champions in a relativly short time frame, and there's a voice and kill display that let's everyone know she's doing her job when she's doing her job.
Very different from Janna where winning the 5v5 sandcastle destruction war isn't done by racking up kills and getting a big thumbs-up from the announcer every time she saves her carry, protects a tower, zones enemies off an objective, does a solid counter-innitation for her allies, etc.
4) Riven is a strong duelist and lacking team-fighter. Very relevant trade-off when a typical game is won by a decisive team-fight later on. Advantageous trade in lower Elo where there's a fair bit of dissorganization, formation issues with team-fights, "team-fights" that are more a string of 1v1 duels where everyone's to spread out to help one-another, and "1v1 me brah" is a tactic a lot of people assume works.
5) Mobility is really solid in a 1v1 fight and can negate any advantage range offers. In a 1v1 battle, taking 25% of your HP when diving on someone while packing a fast-enough combo to OHKO them once you land on them before they can dish out another 25% of your HP is pretty strong.
Mobility is also a lot better than CC in a 1v1 fight. Most CC comes with a long enough casting animation a 1 second stun pretty much stuns the user 1 second as well as the target, essentially accomplishing nothing (where mobility is a very real strength).
In a 5v5 fight though? A 1 second stun affords 4 seconds for everyone else on your team to capitalize on it: bad for a champion like Riven vs. a bunch of ranged champions capable of busting 25% of her max HP apiece for 100%. Really bad vs. other champions like Riven when that means 200% of her HP is just gone.
Also - when traveling at ~575 MS to reach someone allows 1 person on the enemy team to nuke 25% of your max HP, that's bad news when there's 5 enemies that can do that.
6) Mana management issues. Great players can get the timing and ideal scenarios down on mana-draining skills to use them optimally. Unskilled players won't, which makes Riven's manaless nature seem OP (even though Wings does exactly the same amount of damage as a basic AA for the vast majority of a game). All AA's are manaless.
7) Riven's mobility is not kind vs. players that aim where someone is rather than players with foresight that aim where someone is going to be. Unskilled players generally value point-click mechanics more than skillshots, skilled players can abuse skillshots to strike opponents at great range, heard them towards chokepoints and against walls where accuracy is guaranteed, strike opponents in brush/fog of war/invisibility, and trap opponents between a rock and a hard place.
Again - this comes back to a vocal community not having smarts as their known high-point.
8) when you're having issues doing well in LoL, never feeling fast enough is a common concern. You're always slightly out of position, overextending to far, waiting to long to bail out of a lost situation, on the wrong side of your team for an ideal formation. Riven having mobility and skills that play more to the reflexive/responsive wants does make Riven (and all mobile champions for that matter) appear to have an unfair advantage.
Riven isn't OP in the competitive scene (Janna always is - complete opposite end of the specturm), she's just OP vs. the large portion of the player base that the game isn't balanced around by having a kit that assumes ranged champions are gonna use range, AoE champions are gonna AoE, skillshot champions are gonna have some skills, and champions set up with specific team rolls are going to work as a team.
Most assassins, light fighters, and otherwise anti-carry champions suffer from this perception issue as well. Riven's just maintained being the FOTM anti-carry to QQ about ever since Akali got nerffed almost all the way to Evelynn tier.
At one point Eveylnn was the FOTM anti-carry, and then Riot decided to listen to the community and garbage tier was re-nammed Evelynn tier for over 2 years. She was so bad anyone that attempted maining her actually got banned due to getting reported so much, you could look that up if you don't believe me.
Moral of the story though: The community will Q_Q about Riven until she's completely unusable, LoL's mechanics assume you're smarter than that, and Riot's balance team shouldn't listen to them.