Kimbrel when he's not on, starts walking guys and tries to over throw. I think we've all seen it often, and we all saw it on Game 2 (thankfully Simmons saved the day with the tag).
Rivera doesn't rely on overpowering stuff, he wants to get you out as quickly as possible. Kimbrel has stated many times he'd rather have a 5 pitch inning than striking out the side. Problem is, he hasn't learned yet how to do it regularly. I think one of his problems is, he throws so hard that guys can't make contact and end up swinging and whiffing on the first or second pitch. Or his stuff is so overpowering that guys will take pitches instead of trying to hack. With Rivera it's different, guys know they're going to get a pitch and where it's going, and they can't make good contact.
As against the grain as it sounds, I think Kimbrel would be better off throwing 94-95 but getting guys to pop up or flyout, than trying to throw 98-100 every time and getting them to swing and miss. All the effort he uses to try and strike out guys, is why he can't realistically go 2 full innings. Sure his sexy numbers like his WHIP and K/9 ratio won't be historically groundbreaking anymore, but I want Kimbrel to last forever and at the rate he's going he's going to have TJ soon. Rivera to my knowledge has never had tommy john, and he's been about as durable as you can get sans that freak injury.
Anybody remember when Brad Penny started the All-Star game a few years ago? He was clocking 99-100 mph in the first two-three innings because he knew he was only going to be pitching the few innings instead of saving his energy for 6-7 innings.