Nothing:
I was just thinking more like "Hey, but if X fires a multi-shot bow attack, they'd have to roll to hit everyone, so why can't I do it with my darkness in case I get a crap roll on some of them?". I would do it th same way I do in Fourth - listing each target being attacked and making a roll for each.
Well, that's basically how the GM house-ruled it.
With spells, in regular Pathfinder and D&D you'd roll your spell damage, and then combat would need to wait for the GM to make the rolls for the enemies so the other party members know which ones are still standing, and which allies require healing because they stood too close to the blast. Likewise, with your darkness spell as an example, everyone in it would need to roll against your effect every round we're in the area of effect.
By doing it this way, you're eliminating a bunch of bookkeeping because the necessary rolls would be done by the one creating the effect, once, at the moment of creation, rather than tying it to every affected individual's resistance and feats that may or may not apply, and having them roll every round against it. It makes spellcasting more consistent, as either the casting ended up well or the casting ended up poor, rather than a slot machine of who gets affected and who does not per round.