If there's one thing that I feel will hurt Diablo III's longevity, it's the considerable amount of streamlining they did to the skill system. And it appears that GameSpy feels the same way, as they've penned a new editorial suggesting that Path of Exile's deep skill system makes it feel more like Diablo II than Diablo III does:
While the item-hunting loot-pinata gameplay is nearly identical to what we're used to from Diablo and its host of clones, PoE actually changes things up a bit with its skill system, which cribs just as much from Final Fantasy games as western hack-n-slashers. Its passive skill tree is functionally the same as the sphere grid in Final Fantasy 10, with everyone sharing a single enormous map of nodes, the entirety of which can be navigated by characters of any class.
The six different classes each have their own starting point on the grid, which makes obtaining certain stat increases and passive skills easier for each, but there's nothing to stop a bloody-minded Witch from pursuing the Marauder's elemental resistance and regeneration bonuses. The result is a huge number of viable builds, along with an even larger number of completely non-viable ones.
There will inevitably be some flavor-of-the-month-ism cropping up, but this system offers much more than "oh screw it, just make a hammerdin." Skills, meanwhile, are granted by gems which are socketed in weapons and armor and can be further enhanced by modifiers in linked slots, and the result is absurdly complex and nuanced.