It all starts with Diablo 2. Diablo 3 took one path, streamlining and simplifying systems. Path of Exile takes the opposite path, dialing complexity up to eleven.
Path of Exile is a Diablo-style action RPG. There are six classes: Warrior, Duelist, Ranger, Shadow, Witch, and Templar. Each class is locked to a specific gender and model. Like most action RPGs, you wander around the zone killing things for randomly dropped loot, upgrading your gear as you level. However, many of the systems are very different from the standard.
For starters, abilities do not belong to the class. Instead they are gems. All gear has sockets. Sockets are red, green, or blue. You match the ability gem to the socket. Let's say that you have a Templar and you find a Glacial Hammer blue gem. You put that gem in your weapon or armor piece with a blue socket and you can now use the ability. Any class can use any ability, only some classes are more suited to the ability than others. While you have the ability gem equipped, the gem accrues XP and levels up.
Then there is the talent tree. I urge you to go take a look at it. It is the ultimate talent tree, with hundreds of talents. All six classes are embedded in the same tree, but talents closer to the class are more suitable. For example, if you wanted to make a necromancer, the major minion talents are close to the Witch's starting location. But you could still reach those talents from another class, it would just take longer.
I believe the development studio is from New Zealand. This gives the game a unique flavor. The accents are New Zealand or Australian. The native warrior is based on the Maori, I believe, rather than the typical Norse or African. It just feels very different from the norm.
There are other interesting touches. There doesn't seem to be any gold. Instead people trade using scraps of Scrolls of Wisdom, which allow you to identify magical items, and other bartered items. This is a very interesting idea, getting across the idea that this is an uncivilized land without even basic luxuries like coinage. Health and mana potions are gear items which can have stats. The potions refill as you kill monsters.
Path of Exile is also big into "leagues", which are modes of play where you start a new character. Each league often has specific rules. A simple example is the Hardcore league where you can only die once, and then your character transfers to the regular league. But leagues can get a lot more complicated. For example, there's a league which starts at a given time and lasts for 12 hours or so. Every few minutes, the lowest level character is kicked out of the league. The last character standing wins a prize.
There is really only one thing I dislike about Path of Exile, but unfortunately it is a major element. I do not like the graphics. It's all from the "realistic" palette, all dark greens, dark reds, greys and browns. I just find it unpleasant, and even hard to identify elements on the screen. I much prefer more "cartoony" graphics which "pop" on the screen. But this is very much personal preference. Other people may like this graphics style better.
Mechanically, Path of Exile is a very intriguing game. It's Free-2-Play, so it's easy to check out. It's complex, but if you feel that D3 went in the wrong direction, you'll probably enjoy the complexity.