It's not about numbers. Sure, getting the right rolls on items (like all resist or armor, vitality, and so on) are important as well, but GR tiers aren't about those numbers at all. It's more about your gear, gems, gem levels, paragon, your build, ... so many things. It also varies extremely by class/build - many builds are just extremely squishy, and at a certain GR tier *all* builds are squishy. For speed rifting, people go full glass canon and just try to kill stuff before it even reaches you (e.g., multishot DH is the best example here).
It's not about numbers. Sure, getting the right rolls on items (like all resist or armor, vitality, and so on) are important as well, but GR tiers aren't about those numbers at all. It's more about your gear, gems, gem levels, paragon, your build, ... so many things. It also varies extremely by class/build - many builds are just extremely squishy, and at a certain GR tier *all* builds are squishy. For speed rifting, people go full glass canon and just try to kill stuff before it even reaches you (e.g., multishot DH is the best example here).