Path of Exile uses, in my opinion, a very original skill system. I've heard mentions of similarities with one of the Final Fantasy games but to me this is brand new. I have never seen something like this before.
Essentially, skills are broken down into two big categories: active and passive. Active skills appear only on skill gems, while passive skills can be assigned in the skill interface after the character is given a skill point. This happens when leveling or as a reward from quests.
Almost every item that can be equipped contains a random number of sockets (up to 6 for certain weapons, for example). These sockets can be colored red, blue or green, corresponding to the three attributes in the game: strength, intelligence and dexterity. Throughout the game you obtain skill gems (as drops or quest rewards) that you place in the corresponding socket. This allows you to use the skill.
Another innovative thing is that not only can skill gems be moved around to other sockets at will, without penalty, but each gem actually gains experience and levels up, becoming more powerful in the process (notice in the shot above where the Freezing Pulse gem shows Experience... and Level 4). Equipped skill gems acquire 10% of the character's experience so they basically level up with your character.
Since items are not soulbound, not only can you trade armor and weapons but you can also trade high level skill gems if you so desire.
The passive skills are located in a huge and initially scary-looking interface which radiates like a massive spiderweb from a central hub which contains the three basic attributes: strength, intelligence and dexterity. As of now there are 750 passive skills and the character gets a total of 111 skill points that she can distribute.
Passive skills include stuff such as +10 Strength (there are many of these), +x% extra damage, +x% mana recharge rate, +% block and on and on. From place to place there are mini-hubs which branch out but also give a higher bonus when picked (like +10 Strength and +10 Dexterity).
I like this system because it really improves your character with every level. Adding an extra 6% to melee damage, for example, or 10% additional crit, does make a difference.
Hybrid classes will be able to pick skills from both main attributes, mix and match. A strength-based class, for example, won't have reasons to pick, say, intelligence passives.
Going back to active skills, they are also split into 2 sub-categories: actual skills and helpers. Certain items have sockets that are linked to each other by a line. Placing an active skill gem in one of the sockets and a helper gem in the linked socked will apply the helper gem's effect to the active skill. To give an example, if your main skill fires a frozen arrow and a helper skill gives multiple projectiles, then the main skill will fire multiple projectiles. As the helper gem gains levels, you might get even more projectiles fired.
Further, if instead of 2 linked sockets you have 3 or more, you can place additional helper gems (of the appropriate color) in those sockets, giving your main skill even more powerful abilities, like for example multiple explosive arrows.
If you take a look at the screenshot below, you'll see a white item on the ground which has 2 squares next to its name: a red one and a green one. Those are the item's sockets and you can always see them when items drop on the ground. You might also notice that someone is asking a socket-related question in general chat (just ignore the profanity on the next line haha).
Currently I have not advanced far enough to be able to play with these intriguing combinations. I have just got my first helper skill gem recently but unfortunately I don't have a good weapon with linked sockets yet.
The whole system feels very exciting and unique to me and I can't wait to see what deadly skill combinations I can create farther down the road.