Initially Inferno wasn't supposed to be clearable in any reasonable time frame; it was supposed to be a long term, maybe impossible goal for people who wanted a really long grind. After Inferno had been nerfed to soothe the compaints and Monster Power was introduced it became a lot easier to set your own level of challenge. The newer system with Normal - Torment difficulties is a lot smoother but lacks the natural progression path of going from Normal through Nightmare, Hell and Inferno.