To gain skills to use in the never-ending monster-slaughter, your character has to find gems and slot them into equipment. There are dozens of different skill gems, for all sorts of fighting styles: melee, ranged, magic, physical. In addition to the basic skills, some gems provide modifications to the skill's effect. For instance, you might have an archer character using Burning Arrow to set individual monsters on fire, but you'd like it to work as an area effect. Add a Lesser Multiple Projectiles support gem, and now the skills sends out three fire arrows at a time instead of one. That's a very simple example, but the combinations can get very complex as you approach the end game.
As your character levels, you gain points to spend on the passive skill tree. The tree is massive and a little daunting to the new player, but after playing a while things start to make sense. You use points to move along the tree, picking up nodes which provide bonuses to various stats. Usually the goal is to reach one or more keystone nodes that provide major bonuses and/or changes to how the game plays. There are a ridiculous number of choices on the passive tree, making possible all sorts of different builds.
Another unique feature of PoE is the economy - there's no money. Almost every game has "credits" or "gold" or "dollars," but not PoE. Instead, you get "currency" items as loot drops and in exchange for selling items to vendors (or other players), each of which is usable. The currency item effects range from mundane scrolls that can identify magical items to orbs that modify gem sockets to a mirror that can copy an item.
There are two major weaknesses in PoE, as far as I'm concerned. One is a lack of direction for the inexperienced player. In addition to the aforementioned overwhelming complexity, simple things are difficult to find - for instance, if you want to buy gems from a vendor, you have to click on a small numbered tab in the correct vendor's inventory. It's not intuitive at all, and easy to overlook. As you complete quests, more gems may be available there, but nothing tells the player about the new options unless you happen to look. Searching the PoE wiki can get around a lot of the confusion, if the player knows about it...which new people may not. Obviously this problem goes away over time as the new person learns, but I suspect a good number of potential players never got that far. Secondly, the game requires a huge time investment. Changing a character's passive tree build in any significant way is nigh impossible, so trying something different usually means investing hours into a new character. Many skills and important passive tree nodes aren't available until high levels, so you may not even know if your idea will work until you've invested many hours. Once you do get up into the highest levels, keeping your character supplied with maps to clear is another major time sink. The time investment requirements are a major reason that I don't play PoE more.
PoE is a free-to-play game, and as far as I'm concerned they've nailed the right way to run that model. You can buy all kinds of cosmetic items to change your character's appearance. Nothing in the store affects how you play the game, though, outside of additional storage space. Unlocking all the content and playing to the very highest level is possible without spending any money, although I suspect most people do buy some things. I certainly have - primarily that extra storage space.
Every few months, the developers come out with a content update that adds new features to the base game. Those new features are first introduced into challenge leagues, where players must create new characters and use items found only within the league. Those challenge leagues usually run for a few months until the next content update. It's a system that encourages trying new and different things rather than building up one character to use constantly. The newest PoE content update is called Prophecy and was released earlier this month.
I find myself coming back to PoE for a short time on a regular basis, usually to see what's new after a content update. It's such a time-intensive game that I tend to burn out after a while, though. I'll try out Prophecy and see how long I want to keep with it this time around.
