04 Oct 2009 at 13:53 - 2
It's also the skill synergy and size of the mobs. In early proph, you usually find yourself against "groups" of one or two monsters at a time, and they only have two or three skills at most. But later on (especially in end-game Nightfall and EotN) the mob sizes grow retardedly huge, and they have lots of skills to work with, all of them usually synergizing well. Add on to that the henchmen you have to work with if PUGs aren't availible, with skill "synergy" such as Practiced Stance + Ignite Arrows, and I think you've got just about all the ways difficulty increases: Higher levels, better skills (plus monster skills, although PvE-only skills have balanced that out), increased skill synergy, and epicly failing allies on your part.
Edit: If you want to add in Hard-mode, there's also super-increased levels, increased attack speeds, increased movement speeds, increased armour ratings, and increased AI.