The way I usually describe Path of Exile is a stark contrast to Diablo 3 in almost every way.
- Story is there if you want it, and it's not spoon fed or forced upon you - Passive skill tree vs. Diablo 3's - Gems are skills instead of simple stat boosts, and they can be linked with other skill gems to boost or modify the regular skill - Dark atmosphere -- no ponies here! - Dark dialog and style. Surely important characters won't die to butterfly magic in PoE - Goldless economy. Even ID scrolls have value - End game has variety via random maps that open portals to random dungeons. The maps can have modifiers like +20% item quality, or take constant chaos damage. Contrast this with Diablo 3's end game where you grind the exact same exact areas over and over without much randomization at all - Crafting is meaningful though costly, and in my opinion, rightly so - PVP is in - Competitive and rewarding ladders with interesting game modes like cutthroat where you can PK anyone and take their items - Ridiculously passionate developers who are ALWAYS in touch with the community. It's not uncommon for the lead developer to drop in and chat with players from time to time - Lots and LOTS of viable builds. While there are always a few cookie cutter builds, there are just way too many viable options it's mindblowing. Definitely not spin2win like in Diablo 3 - Developers are very much against pay-to-win, but extra stash tabs are really cool =) - Free vs. $60