Probably the stupidest thing, but I like how FFXIV has better taught me when to use aoe rotational skills over single target rotational skills (I'm purposefully using the word 'rotational', as I'm referring to skills you use in rotation as opposed to CD damaging skills).
I think it's the fact that the tooltips uses flat values for potency and diminishing values in secondary targets, while WoW uses expected damage based on the calculation and just says "damage reduced beyond x targets".