Her mobility (e.g. when being a jungler) as she can easily escape without being forced to use Flash. Her AD scaling shield and ranged execute are pretty annoying combined with her overall great amount of damage and CC (knockback and stun).
I think the whole topic of "hate" evolved around Riven being an annoying matchup in lane as many people (used to) main this champion. I don't really think it's an outstanding amount of "hate" towards that champion because there are many other champions considered to be objects of hate such as Teemo. It's just the whole theme/ design of champions or maybe the meta-caused massive overuse (every game there is a player of champion XY) creating the impression of that champion being hated by literally everyone. The attitude of Riven players, Riven streamers and "Riven Top Plays" also come to my mind.
Oh god, how many times ?
Ennemy Riven kills you : "Outplayed, noob"
You kill ennemy Riven : "omg lag" "pff ganks noob" "pff op champ" [insert dumb excuses here]
I personnally hate her ult, more than the rest. AoE Ranged Garen ult on low CD + AD steroids. Yeah right ...
I think the whole topic of "hate" evolved around Riven being an annoying matchup in lane as many people (used to) main this champion. I don't really think it's an outstanding amount of "hate" towards that champion because there are many other champions considered to be objects of hate such as
Ennemy Riven kills you : "Outplayed, noob"
You kill ennemy Riven : "omg lag" "pff ganks noob" "pff op champ" [insert dumb excuses here]
I personnally hate her ult, more than the rest. AoE Ranged Garen ult on low CD + AD steroids. Yeah right ...
btw Riven players are still less annoying then the greedy Kata players.
Why do People Hate Riven?
Let's analyze this sentence first.
People Hate Riven?
Hate Riven?
Riven
Here is the answer you were looking for.
They see her jumpin', they hatin'.