One of the best poe currency concepts in Path of Exile is its own passive tree and ability gems. Each time you level up, you unlock points which you can use to uncover nodes in the passive skill tree. Minor nodes offer slight updates to your stats, even while major nodes unlock strong passive abilities that fundamentally change the way you perform.
This works in concert with skill gems, which slot to parts of equipment and give your character spells and skills to use in battle (unlike Diablo 3, that they do not have any provided skills). Items will frequently have multiple stone slots, and at times they are linked together. When I set an ability like Orb of Storms to a slot and then join it with Curse on Hit, Flammability and Vulnerability, every moment that my Orb of Storms hits an enemy it'll also use both of those curses to them. It is essentially a highly robust spell-crafting system.
Nevertheless, it's when skill gems and the passive tree float that things get really exciting. My Necromancer relies solely on Scorching Ray to cope with damage. Obviously, I have unlocked lots of nodes that increase fire damage, but some big nodes possess these clever synergies that are really fun to unlock. Take Elemental Equilibrium for instance, which makes enemies I struck with elemental damage temporarily receive 25 percent immunity to buy poe chaos orbs all those components and -50 percent immunity to other components.