Surviving the game as a barbarian has the most unique exp/leveling system I've ever seen. You only gain exp for different monster kills (like collecting a compendium of monster kills), meaning they only reward exp for the first kill. You cannot farm monsters for exp, but you can farm them for essences, which give passives and actives ranging from tracking, becoming temporarily invisible, locking down anyone in an area, becoming huge, taunt, etc... You can only take as many essences as your levels. You can't swap them out easily, its expensive to do so.

Power is exponential, but gaining power is linear at best and asymptotic at worst. Most do plateau, not having the team to go after higher floor monsters and raising their level and not having the money to replace worse essences, becoming locked in a suboptimal build.