The Reason Riven is "OP"
It's because she has a mix of cc, damage, and mobility. You can't escape from her, and you can't catch her. If she's coming towards you and hits you with her third Q, she can immediately follow up with her W and keep you locked down long enough to unleash her whole combo(on your entire team, all of her damage is AoE). If she's behind, she can just farm back into the game, and she has no resource(although that's changing soon).