copy pastaing a more detailed game overview because zenny isn't special
(disclaimer: Honestly the main site does a good job of hitting the game's high points in design and I recommend just looking at it, if you are a diehard ex-D2 junkie it will intuitively make sense almost immediately)
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OK so basically this game is Diablo 2 at its core, action RPG, more clicking than Zenny's jerkoff sessions, randomized magic item loot is the giant appeal along with character progress and min-maxing your preferred build with a handful of skills that you center your char on. On the flipside the skill diversity and overall complexity is several magnitudes more indepth and a lot more detailed. BUT it is still D2. If you didn't waste hours of your life playing D2 you're not gonna like it. Period.
There are 3 stats and 6 classes. The classes ONLY differentiate on three ways: starting stats (and which stats they are supposed to emphasize), starting location on passive skill tree (sphere grid), char sprites. They are Marauder (Str), Ranger (Dex), Witch (Int), Duelist (Str/Dex), Templar (Str/Int), and Shadow (Dex/Int).
Stats govern your build. All base equipment and skill gems have a prerequisite of either one or two stats, in the aforementioned possible combinations. There are basically Str, Int, and Dex gems, though a handful of them are hybrid (require a secondary stat). Same goes for equipment types (Str armor is Armour-boosting, Dex is evasion boosting, Str/Dex boosts both, etc...) The passive skill tree is the only thing that raises those stats, aside from finding equipment that raises them as well.
The consequence of this is that you can do some really fucking elaborate off the wall shit due to how easily you can traverse across the passive skill tree and pick up shit that you wouldn't intuitively link to the class. You just need to map out a path that hits the nodes that allow you to pull off your build.
http://www.pathofexile.com/passive-skill-tree is the skill tree in question. It is fun to just browse and of course you can theorycraft to your heart's content on it. In-game features an equally fully functional version, of course.
Skill gems are the other interesting part of the game. Aside from being the method in which you actually use active skills, you can find support gems which is where the system really shines. Take a spell for example,
Spark, which shoots out a scattershot of random electrical bouncing orbs. Spark is classified as Lightning, Projectile, Spell, and Duration. This means that any passive effects (whether from the passive skill tree or from the equipment) which refers to these types (i.e. +50% lightning damage, +30% faster projectile speed, +10% spell duration, etc.) applies to Spark. This also means a support gem like
Greater Multiple Projectiles (which halves damage but turns 1 projectile into 5) or
Fork (which causes a struck target to produce two new forked projectiles upon being hit), or
Lightning Penetration (which ignores a certain percent of a target's lightning resistance) can all be combined with the ability, provided you have the gems and equipment that have linked slots to allow such a combo.
There are some unintuitive combos that have some pretty interesting effects.
Added Cold Damage, for example, looks like a simple skill that just adds the ice element to the skill's damage. But when you combo it with something like
Raise Zombie, this actually causes your summoned undead horde to all inflict cold damage on their attacks, a useful trait given it yanks the chill status effect from cold damage right from D2.
Aside from that, the game currently features 3 Acts (the last of which is as long as the first two combined), three current difficulty modes, and an endgame feature called "Maps", wherein you can create instances of randomly generated dungeons with high-end bosses and loot from droppable Map items.