Resists cap at 75% just like most other games.
Enemies have innate resistance penetration equal to the zone level up to 75 max pen.
The resistance cap is calculated before the penetration is applied so on zone level 75+ your total max resist will be 0. Monsters in the game are balanced around this system (so are weaker in general than PoE). This means that having less than capped resistance will result in less increased damage taken compared to a traditional resist systems.
To make an example for clarification, in PoE you will take about 4% more damage at 74 resist compared to 75 resist. In LE, you will just take 1% more damage.
Resistances are still at the top of the defensive stats because they mitigate both hits and dots. Most other defensive layers do nothing against dots.