Path of Exile has three unique gameplay mechanics that set it apart from other Action RPGs: its passive tree, the customization of skill gems, and the economy.
the passive tree is a tree of over 1,300 nodes that every character shares. the nodes provide benefits from the mundane (+10 strength) to the exotic (if Health isn't full, automatically sacrifice a percentage of Mana to recover it). Your character gets up to 120 points to work their way through the skill tree, so they won't be able to get EVERYTHING they want. The customization of the passive tree lends to the game's replay-ability: do you build your character to be a glass cannon, a tank, or a little bit of both?
Skill gems are items that grant your character their skills to use when equipped. They get augmented through support gems, which can modify skill gems to make unique and powerful combinations. While skill and support gems are level gated (you can't use some gems until your character is at level 38), any character can technically use any skill if they select the appropriate passives on the tree. You can have a Witch that gives up the spells for a warhammer. You can have a Marauder that forgoes beating on things with a sword and shield and instead opts for laying traps for enemies.
A huge gameplay mechanic that sets Path of Exile apart from everything else is its lack of in-game currency. There is no gold in Path of Exile. Instead, players trade powerful items with each other for expendable currency that can modify equipment. There are over 20 different types of these currencies, all of which modify items in different ways. A few of these items have led the pack as trading standards, with players exchanging items for Chaos Orbs (popular), Exalted Orbs (hundreds of Chaos), or Mirrors of Kalandra (lol you'll never see one of these).
For a free-to-play game, the lore for Path of Exile is surprisingly deep. The main story sees you as a shipwrecked prisoner who has to fight their way through Wraeclast, and ends with you becoming powerful enough to slay the dozen-odd different gods that are giving everyone a hard time (that's not a spoiler, literally every action RPG turns you from feckless nobody to total godslayer). On top of that, there is a thousand-year-old history of previous civilizations and their usually violent ends there for you to discover. Making your way through this landscape and history gives Wraeclast a feeling that it has been lived in, and gives you a chance to do more than simply "kill stuff and get loot to kill more difficult stuff."
If you haven't played Path of Exile before, it is a remarkable action RPG that's with your time. If you've played it but it was years ago, the new content is definitely worth a revisit.