I'd say the biggest problem with Riven is her mobility. Get rid of those dashes that go with her Q and E and we've got a much more balanced champion. Once she has a bit of CDR it's impossible to catch her... the only time you really get to fight Riven is when Riven wants to fight you... and that's pretty stupid.
I played Udyr with MS quints and level 2 boots and it was impossible to catch her. It's like yeah you bear stance flashed at her... but she just E QQQ'ed her ass half way up the lane so... yeah. She started at our inner turret mid, I bear stance and flash practically out of the bushes by raptors but riven just QQQ E spams laugh QQQ E and I give up what is obviously a hopeless chase.
There is literally no other champion in this game that brings out that sweet toxicity more than Riven. Most other champions when you get owned by them you can go okay I misplayed here or there, I could've outplayed this or that, juked here when I didn't...etc... With Riven it's like okay.. she's F*&%ing broken. Even after they fixed that bug that like EVERY riven player used and abused to reset their Q CD... she's still broke as hell imo.