I think the biggest appeal for Grim Dawn is the map design. I almost never get frustrated with it the way I do with other ARPGs.
I know grim dawn gets constant praise here but still.
When I play other APRGs, I often get bored relatively quickly and get a sudden urge to quit. For Grim Dawn, I will have no problem playing for 5, 6 hours straight.
I think part of it is the map design. Randomized maps come with this overwhelming feeling that any turn can be the 'wrong' turn and you will have to backtrack a while. You never truly feel like you're going the right way, because there is no possible way to know if you're going the right way. It adds to this sense of dullness and lack of curiosity when going through the maps.
With grim dawn, there is a sense of purpose to every single location. You don't get truly lost often, and if you do, there is usually something unique to find wherever you go. The stress of getting lost or having to backtrack or the feeling of placelessness isn't really there.
The big downside of course is doing the campaign again on alts. You've seen it all before. But that isn't really too different on, say, PoE2s campaign, where the maps are mostly the same, just with different paths.