Riven's animation cancelling is a problem
It's no secret that Riven has had access to animation cancelling for a while. 6 months ago, when patch 5.1 came in it gave Riven access to one of the most broken mechanics in the game. The following video illustrates how it works. What this allows Riven to do, is attack much much faster than intended, and propelling her burst through the roof if she gets the drop on someone with her Q.
Just a quick one for everyone: Meddler stated that the Riven changes were an unintended side effect of the CC overlapping system released in that patch. Well, 6 months later and we're still waiting.