Riven is in a bad position right now. I have a lot of games played as Riven (probably around 500 since release?) so I think I have a good idea of her current state. Since I don't want to sound like a lone nut, I do want to point out that Riven's win rate (as shown on lolking.net) demonstrates that something is off. Her win percentage fell almost 10% (big drop!) since the start of preseason 3 to now. She even was in the "ten lowest win rate champions" 2 days ago on LoLKing. Let me preface this by saying that I know people will argue that win rate doesn't mean everything. I do agree that win rate isn't indicative of a champion's viability, but the sharp decline in a champion's win rate in a period of ~1 month shows something is wack.
Here's my assessment:
First of all, Riven was nerfed pretty hard. Her ultimate cooldown was increased by 50% at level 1 (35 extra seconds to it) and her base HP5 was cut in half. The patch notes for this can be found here.
The HP5 adjustment isn't really that problematic since the increased Valor shield makes up for it, however, the ultimate cooldown change is huge. The new ultimate cooldown makes it nearly impossible for Riven to lane against certain top laners (such as Darius). Before, Riven could outplay champions (like Darius) with her lower ultimate cooldown, but she can't really do that anymore. Furthermore, the higher ultimate cooldown really hurts her potential to snowball the early game. While I do agree that the ultimate cooldown Riven had was really low, I think she needed it to remain viable in top lane. Regardless, Riot increased the ultimate cooldown way too much without any form of compensation. At the very least, Riot should have given something to Riven's late game if they took away so much of her early game.
Next, several of the core items Riven builds into have been nerfed. These were indirect nerfs to her, but still impact her in a big way.
- At the start of pre-season 3, GA got hit pretty hard with nerfs (resistances were lowered, but item price wasn't). GA was a core item on Riven (it synergized really well with her shield and lifesteal).
As of patch 3.01 (most recent):
- Bloodthirster now costs more.
- Blackcleaver is in a weaker state (less health, more stacks required).
Finally, season 3 is known as the "health meta". Riven doesn't really scale that well with health such as other bruisers. Riven also has really good AD scaling on her abilities so she is typically forced to build glass cannon to make the most use out of her kit. This is problematic in this meta.
Basically, the changing meta in addition to the nerfs Riven received has really impacted her performance harshly. I don't think that Riot was trying to make Riven useless in this meta, but that's what they did with the previously mentioned balance changes.
My 2 cents.