Quote from Zunori;1275087:
I don't understand why people don't play Riven mid a lot. She stomps practically everyone but Annie and LeBlanc in lane and if she can snowball off that then she becomes a monster.
Here's why: Her kit is strongly towards snowballing, she's not your safe laner.
Riven generally struggles against match ups with either high pokes or good sustain. Cho'Gath, Quinn, Hecarim, the list can go on.
To utilize her level 1 or level 2 damage, you need to be playing against melee champions or range champions with low kiting ability/low poke. Pre-rework Nidalee was an example of this. Even in such cases, there are some champions of the above categories that she does not necessarily win out on. Zed is a primary example, even though Riven will beat out Zed later on, his level 2 is more potent than Riven, unless Riven can make Zed waste his cooldowns and land stun on him.
With all that being said, Riven is highly a situational pick. People at higher elo will know exactly when and how to punish Riven for her misplays, mispositioning, etc.
In general, Riven is not really in a good meta right now, because of tank meta, but that's not to say she's in a bad champion. In solo queue, not everyone will follow the meta, and if it's a match up that I can play Riven in, I will play Riven. I usually like to play Riven against Rengar, Yasuo, Katarina, Zed, Talon, Cassiopeia, or just play jungle Riven whenever I feel like it. I'm not a usual Riven main (as in she's not really one of my most played champions), but I do maintain 80% win rate on 40 games on Riven.
Also, her kit is HIGHLY dependent on her team. There are usually two ways you can build Riven. One involves Last Whisper, and the other doesn't. With the recent Black Cleaver changes, you can no longer hold on to your Brutalizer to go into Black Cleaver. The first build would consists of Ionians, Hydra, Youmuu's Ghostblade, Last Whisper, Merc Scimitar, and Last item, usually a Defensive item (Most likely Randuin's or GA), or otherwise, Frozen Mallet or IE. The other build would be Merc Treads, Hydra, Black Cleaver, Trinity Force, Randuin's Omen, Banshee's/Visage. As Riven, you can either initiate with Flash Knock up/Stun, or just follow up, and both of these cases show dillema. When you want to initiate, you'd want to build a bit tanky, in which case, you'll lose out on pretty big damage, and you still won't be as tanky as say, Sion. When you want to follow up, you want to have a team that is super tanky, otherwise, your team will most likely get initiated up on, and you can tell the outcome. Generally, it's best bet to play her top lane, where she has more favorable match ups, able to split push and pressure, and snowball from there.