And then there's the passive tree. 1300+ passive skills, allowing you to build your character in a way that is really, truly unique, making a build that ONLY you have, and custom-tailored to the skill gems you wish to use. There are just enough respec points to fix clumsy mistakes, but nowhere near enough to change roles with the click of a button. It rewards planning and diligence without overly punishing mistakes.
Progression and loot feel awesome, with 100 levels to go through. This means you're constantly leveling, just like in D2. It also means items aren't designed just for the cap. There's no gigantic gap between level 50 and level 60 items. Sometimes you'll find a godly level 1 unique that can last you the whole game, or that you can build around. Gear can convey game-changing bonuses like converting all your block to spell block, or making you cast spells from your life pool, or giving you stun immunity but removing your ability to evade. They rewrite the rules of the game and make your character a whole new sort of class just by putting the item on.
Ladder is back and the hardcore modes even tell you when top players die. There are cool new game modes which add new mechanics and they're introduced every few months. Free new content and enemy types. Maps are actually random unlike D3's "chunks" in a 95% static map. Exploring is fun and you are never certain where waypoints or exits will be. Even bartering feels better, you actually communicate and trade with other players, can haggle over deals, and the whole game just feels fresh and alive with constant updates and info straight from the developers while D3 just feels like a dead husk of a game being thrown an occasional bone by PR puppets.
Path of Exile is basically the Diablo 2 sequel I was hoping and waiting for over a decade. I guess there is not much else to say in this post except I wonder where it all went wrong.