Champion and Rare monsters have special abilities that enhance their difficulty. When I was playing on Normal difficulty, these mobs always had one special ability. On Nightmare, I've noticed that they always have two special abilities. Does this continue through all the difficulties? I've found some really nasty combinations of just two abilities, and I can imagine some horrific combinations of three, much less four of them.
Champion and Elite mobs gain an additional modifier for each difficulty tier (so 2 in Nightmare, 3 in Hell, and 4 in Inferno.)
Get ready to enjoy Molten Vortex Jailer Fast!