Prepare for the penalty
Nightmare applies a −40 resistance penalty; Hell applies −100. The default maximum resistance is 75%, with a separate hard ceiling of 95% for raised maximum resistances. The captured resistance clamp bottoms out at −100%.
Crowd control gets shorter
Chill, freeze and curse duration divide by 1 in Normal, 2 in Nightmare and 4 in Hell. A long control effect in Normal therefore does not imply the same safety window in Hell.
Other combat settings
Related: Defense, resistance & survival · The five-act route