What changed
In the current Forever beta build the Gnome racial Eureka! has been nerfed once more. It no longer works on periodic effects at all, and channelled spells do not count as periodic. The first nerf had cut the resource discount by a flat percentage.
Effect per resource
Eureka! now applies to your next 3 non-periodic damaging abilities: they cost 10% less Energy, Rage or Mana and deal 10% more damage. Healers get the effect on their non-periodic damaging or healing abilities. The previous 10% effect on periodic damage and healing has been removed.
How hard it hits each class depends on how many of their spells are periodic. The build's known issues also state that Eureka! does not yet cover every damaging or healing spell for some classes, which a later build should fix.
Original report: Wowhead