From personal experience I see that every top had its limits to there power (resource management, high cooldowns, etc.)
What I see in Riven is that she has it all, high mobility, passive tankiness, no mana or any resource at all, high damage, scales great, good at all phases of the game, hard to gank, good burst, no high cooldowns, and strong team fight presence.
Am I missing something here? Jayce had and has his limits, and so did every top. But Riven? All I see is lack of harass isn't that a bit much to put in all one champ?