#3 is very streamlined/simplified compared to the first 2 (and those two weren't exactly Path of Exile). Stats are gone, skilltrees are a lot smaller, etc. On the plus side, the game is still fun. Its just really simple.
I have mixed feelings on what they did to the fame system. In the first 2, leveling fame made the game more fun in between leveling (don't underestimate how much more fun it adds getting extra skillpoints dropped here and there). Now with each fame level, you get a random reward - can be crafting ingredients, items, etc. Its not bad or anything, I just prefer the old system.
Relics is just another skill-tree now. The Legendarium gives you 3 extra skills from breaking down legendary items. Unfortunately there doesn't seem to be that many legendaries in the game compared to say... Grim Dawn.
Now they said they're going to add more content post-release, but if the prospect of a very simple/streamlined action RPG seems unappealing, I'd probably give it a miss. Its still fun and I don't really regret dropping money on it though. Its a good break from some of the more complicated ARPGs out there. Bear in mind I avoided Diablo 3.