I know we have had some limited discussion of this game before, but with a new expansion on the horizon (Forgotten Gods), and this forum's general love of the loot-hunt, ARPG genre, it is really time to give this game it's due.
Grim Dawn is without question the main successor to Diablo 2 in every way. Many people (especially here) actually prefer Titan Quest to Diablo 2. Well, Grim Dawn takes the best aspect of Titan Quest (which in my mind is the dual-mastery system that allows dozens of class possibilities) and eliminates the glacial pace of that game. It's basic premise is based on Titan Quest, it's engine is based on Titan Quest. But it is faster, it is tighter, the monster density is infinitely better. There are more secret areas and dungeons than you can shake a stick at.
The core of any game like this is the skills and the loot hunt, and this game has both in spades. You have your main mastery, your supplemental mastery. Then you have a Devotion tree of dozens of Constellations to further certain aspects of your build. Every piece of gear can be enchanted (in this game called augmentation). If you liked the resistances and the role they played in Nightmare and Hell in Diablo 2, you will be happy to know that Grim Dawn has at least twice as many you need to manage to survive. There are more Legendary Items and gameplay defining sets than one can count. There is a faction and reputation system. The more of certain types of enemies you kill, the more lucrative spawns start appearing. There is a wave-mode called the Crucible if you need to get yourself a level or two and some quick loot. It currently has six acts and is expanding to eight. It brings back the idea of farming certain bosses for certain items. Picking up gear is necessary because gold (iron bits in this game) is scarce and absolutely necessary and meaningful to your success, yet thankfully your inventory is always big enough that you aren't constantly porting back to town. Crafting materials matter. There are a myriad of potions that give actual buffs and resistances.
Bottom-line: The game is accessible, deep, fluid, and best of all, it is a totally solo experience. While you can certainly team up with friends, no ARPG (outside of maybe Torchlight 2) has ever been this solo-friendly. The fact that the drops aren't balanced around thousands of people playing means you are always finding something exciting and making progress that doesn't rely on other people. This game has always been good ever since it was released, but they have not only vastly improved optimization, but the expansions (and coming expansion) have fleshed out all it's weaknesses and turned it into an absolute juggernaut of the genre. It combines the best aspects of Diablo 2, Titan Quest, Torchlight 2, and Path of Exile and has very little of the weaknesses of any of those games. There is no reason any ARPG fan shouldn't have a copy of Grim Dawn on their computer. On top of all that, the developers are totally focused on listening to the hardcore community, and even a cursory visit to the forums will show evidence of this. There isn't any real toxicity in the Grim Dawn community, just a couple thousand people who love ARPG mechanics. It's turned into one of the great games of recent years. If you aren't a person who gives a crap about ladder races or online economies, there is no other choice for the king of the hill.