Did I say riven didn't have high outplay potential?
She has:
Damage
Mobility
No mana
A ranged execute
And an amazing shield dash.
A lot of champions have damage and mobility. Having "no mana" isn't by itself a notable thing: It still comes down to what their kit does, being manaless doesn't mean anything in and of itself here.
She's not that difficult to pick up, but some people can do some next level shit. I've played her.
To perform her well enough to be at the point that she's really strong, yes, she's a bit harder than a lot of other champions. Facerolling and just spamming her skills will cause you to lose the bulk of her damage. She flows well in play and has relatively straightforward skills (/a forgiving shield) but she has a relatively high skill floor.