Riven sux.
Hard.
While her damage is amazing, she has almost no survivability, windslash is almost completely pointless because of its speed, and she gets hard countered by ANY armor building. Her spells take too long to cast in comparison to other champs, her dash is so short it cant go through anything, her attack range is pitiful until ulted, her shield is ignorable, the stun cast animation is about as long as the damn stun, longer if they have ANY tenacity. She's just so under-par with what I'd expect from what seems to be a pseudo-burst/dot ad champ. She's not exactly bursting, but she's not exactly damage over time either, but she has the survivability of a burst champ, with the play style of a damage over time, which means she dies before she can actually do anything.
Plus her voice actor sounds like a whore who is just about to break down after coming to terms with her sluttyness. Seriously.
Good god, she's the first champ I'm actually starting to regret buying with IP.
tl;dr: Riven sux, sounds like a slut, squishier than a wet paper bag, poor damage output compared to similar champs.