Also no Combat XP will allow PoE to have ACTUAL difficulty levels, unlike what all other RPG's have nowadays (including D:OS) just affecting Hitpoints and Damage.
They can tweak encounters... easy will give less enemies or less powerful (with combat XP this would also mean less XP, eventually making Easy less easy)... Hard could feature more and stronger enemies without risking that those enemies higher XP makes Hard easy.
Combat XP can also be given for the entire encounter, no matter how it is scaled to the difficulty setting, and not just per creature. Sort of like how it is done in PnP.
It's up to the designer, quite a bit is possible if he isn't lazy and is also competent.
