As I've only had a few days to play Grim Dawn vs. Four Years this might be a bit premature to say but. Remember that "feeling" you got when you played D2(Two no typo), when it always felt fun no matter what act you were in or what you were doing and you just couldn't get enough. Grim Dawn brings back that feeling. It's probably in short called addiction. Anywho, I just can't play D3 for longer than a month or two after every patch. Someone said there is only an illusion of diversity, which is true but I"m starting to think it's an illusion of fun.
So far what I've noticed with GD is that it has a little of everything I liked from recent and past Arpg\MMO. D3 on the otherhand is stuck trying to be the next big thing. For myself it's not lasting long enough. I'm not here to bash on D3, I like the game, just have to be honest when it comes to my idea of overall fun. D3 is fun, it has it's points that make you enjoy it. It remind me of a sucker, you can only lick it so much then it's gone. How long it lasts depends on how fast you can eat it. I'll be back next patch I'm sure. For now, rather once again I'm gonna play something else till a new patch comes out. If you feel D3 is also a bit stale during Season X or Patch 2.XX. Give GD a try. Might help relieve some of the WTF is going on tension that seems to come after a patch.
See ya next season.. Enjoy your time no matter what you play or what you do.
P.S. I do not work for any developers. I'm not trying to sale you anything but my opinion - it's on the house =)