Riven is strong, but she's not as strong as you think she is.
Sure, she is really annoying to lane against because she can really punish someone for making a mistake. She is also annoying to play against when she is ahead, but that applies to any champ that gets ahead.
I don't think she needs a nerf. She is just a strong champ that--when played correctly-- can be annoying.
Same thing for LeBlanc. Just like BluePolarizer stated, She is a champion that can instant burst and escape with no problem. Sure, this seems really "broken" and OP, but that is only when she is ahead. A Leblanc that suffers thru the laning phase will not be problematic--or shouldn't be.
Idk. I guess what I'm trying to get at is that these champions are really strong and annoying if they are ahead or played correctly.