What Riot fails to achieve when it comes to balance is making tanks win fights not by out-damaging, but by OUTLIVING opponents.
Let me put this into an example to demonstrate my point.
meets
in the top lane (we're talking season 5 and 6 scenarios here).
Riven has an explosive level 1-2. She tries to go aggressive, but Shen keeps his distance, losing out on a bit of XP but the wave pushes towards him. He lives until level 3, when he hits a small power spike in trading potential. While farming near his turret, Riven tries to poke him with abilities. She slips up, not realizing that she's pushed up and that the enemy jungler has flanked her and blocked off her means of escape. Shen taunts her and she dies.
5 minutes later in lane, Shen has a gold lead. He has bought himself
because he has been CSing and he is level 7, having ganked bot lane and teleporting back top just in time to save his turret. Riven has a
and is dishing out much more damage than if she had bought any other item. However, while using her combo she is about to use her 3rd Q to knock him up for the combo when Shen responds with a swift taunt, pulling his spirit sword through her and whittling her down. Riven tries to reciprocate, but she has lost her opportunity to deal damage and her cooldowns, while relatively short compared to other champions, will not come back up in time for her to be able to break through Shen's bulk.
Shen wins the trade, and Riven is forced to either flee or die, because she cannot kill Shen before he kills her with another round of empowered autos, because she has little to no bulk being a fighter and not a tank.
######(Sorry for using Shen as an example, every other tank champion I've played top against Riven has insane disruption and/or complainable amounts of damage.)
Nowadays, the game has seen hits to survivability, significant buffs to damage, and as a result tanks have become "your damage scales with tankiness or you are irrelevent." Assassins and burst mages used to be the only champions that could throw out a combo (usually when fed) and kill their squishy targets with ease. However, now that is no longer unique to the class, they just do it so fast the game has become uninteractive.
Everyone has become Rengar.