I second MaxTheLimit's description. The passive skill system is probably a bit more complex than anything that was in Diablo II, but I don't have a lot of experience with D2.
Also, so far they are sticking to their guns of not making it "pay to win". The only updgrade you can purchase that will have any effect in game is increasing your stash capacity (note, this is not your personal inventory capacity, as this would imbue quite an advantage, but rather the capacity of your storage chest in the "town" zones). The rest of the upgrades are purely aesthetic.
I'd like to get back into PoE to try out Act 3 at minimum. I haven't played since last fall, but it was a pretty well polished ARPG back then and sucked more than a few hours. I'll see if I can scrounge a few hours over the next few days...
It will be interesting to see how jammed the servers will be (they've already placed orders for more...)