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I'll sum it up for you below.
To everyone who's played Diablo 1&2, imagine a game with Diablo 1's class flexibility and customisation and combine it with Diablo 2's combat system. You now, roughly, have Path of Exile. Sprinkle liberally with the most depressing themes of Diablo 1&2 (defeatism, evils wins, etc) and you get the basic setting concept.
Add onto that some genuinely interesting features like custom ruleset leagues (that people can purchase and set up, like having your own mini Battle.net) and a currency system based around bartering (trade basic items to get identify scrolls, use those scrolls to identify items and sell those items for orbs of alchemy, either sell those orbs to get better orbs that allow you to upgrade equipment or use them to upgrade your equipment), and you see the really amazing game beneath it all.
Oh, and just look at the graphics. It's basically Diablo 2 in 3D with the most gorgeous polish possible.