tanks and bruisers have to initiate, know when their team should win the battle...and jsut get all amongst the enemy and be as annoying as possible, daring the enemy to kill them first (which usually will happen)...you cannot care about you kdr as a tank...only worry about whether teamfights won or lost...and why.
bruisers usually focus the enemy squishy carries...they can actually get in there and live long enough to do that, while the ap/ad might not be safe entering the range of enemy carries.Pure tank types just do whatever they can...not as much of an issue who they target...they jsut are there to protect the carries and have strong engagement/initiation.
Its very common in my experience for tanks to be terrified of turrets. Dont be. ignore the stupid turret and dive the hell out of people trying to protect themselves with turrets. You have to trust your teamates will assist. If they dont, it will look like you are jsut feeding. But you werent. it was their fault, whether they know it....its painfully annoying and stupid, as an adc...watching our whole team of 5 be unable to attakc 3 ppl defening a stupid turret while other teamates fo theirs slowly backdoor. Thats extremely bad tank decision-making. A turret isnt goign to upset the outcome of a 5v3.
comparing riven and amumu...amumu will wait and see if his teamates aren ear and look ready, then initiate hard with ult, and jsut do as much as he can while keeping an eye on carries to go assist them with cc and such on bruisers attackign them.
Riven woudl wait for amumu to ult, and then quickly dive onto the ap/ad and at least chase them out of the teamfight.