Riven's health regen has been nerfed which makes poking and kiting a good counter if you have some range. That is, her sustain is lower now.
She can't go head to head after wearing her down. Even if you're playing Teemo top lane
Bottom lane, a typical ADC and support will poke and kite her out of lane.
Patch notes: http://eune.leagueoflegends.com/news/preseason-balance-update-2-patch-notesPatch notes
