and since Riven can't protect her ADC, well...
People keep talking about a tank meta, but I think of it as more of a balanced meta. I still see assassins and mages (case in point: OP played Riven) being played, but most of the frustration I see is that people can't nuke or assassinate tanks (which is the effin' point).
To me, I find success in considering team comp synergy over carrying potential. Instead of running three tanks like people apparently thinks is the dominant strategy, I find that one tank, one bruiser, one apc, one peel or engage support, and the right adc for the team (caster and burst for engage, utility for disengage, hypercarry if both are present) is the most effective.
I don't understand what the big deal with the meta is. It's all about positioning. You ward the jungle, avoid the cc during ganks, let the adc focus the tank while the team protects them (people ALWAYS forget to protect the adc while chasing a low health enemy, then get destroyed from behind), and focus whoever is in range of your abilities (assassins use mobility to get the enemy adc/apc, then escape, mages abuse their range, blah blah blah). If you ignore your adc and focus solely on getting a bunch of kills for yourself, of course you're going to lose to the "tank meta."