One thing I forgot about running D2 is the sharpening filter for Nvida graphics card also has a noise removal. I turned that all the way up as well. The animation frames are very crude and there’s alot of clashing of textures, for example, in character animation, etc.
Now concerning Reign of Terror:
I think when I was playing ALOT, nightmare and normal were quite difficult, but I was non melee so I did well, and I’m used to hard difficulty; but it was impossible for some character types.
I think how much I play this mod will depend alot on my impressions of depth and easy access as Diablo can do at it’s best. A great aspect about Grim Dawn is that crafting can be done by anyone without looking at databases with the exception of not finding specific items for recipes, and that transitions well into fixing what I thought was the biggest flaw of D2, the Horodric Cube. That takes care of itself. D2 mods like Z-El scare me with 50,000 recipes when in most games I only use it to store loot so I don’t have to go back to town. But again, that’s not an issue for me in Grim Dawn and I rather enjoy the system, and this mod seemed fine there as well (even if in the base game sometimes it takes hours to actually get what you want). It just all seemed worth the effort and the reward was justified in that effort.
The other issue is the concept of removing the passive skill trees (forget what it’s called currently, LOL) that add additional skills to base skills and give the game alot more customization and ability to alter builds. I’m unsure how that would affect my love for the mod or dislike for it, if it were removed. I guess it depends on how true to the original game people are looking for, and if that’s a distraction to them or a welcome addition into the D2 universe. Last I checked more people wanted these constellations removed. And all of that depends on how well crafted the skill trees themselves are in the final product, and if they have many valid and interesting builds. My personal opinion, despite the constellations feeling out of place in their current form since it’s built around GD and not D2, was that it helped make up for the fact that the D2 skill trees are so much more simple than GD skill trees. While I love this mod, playing it helped me realize how simplistic the synergies and builds are compared to the evolution of the ARPG since. One of my favorite aspects of this mod, initially at least, was the constellations along with the D2 skill trees. But my concept may change with that in time.
I still have not yet come to terms if I’d enjoy playing a Grim Dawn class with a D2 class, but I have no issue with them being in the game. Just like with Grimarillion, so far I’d decided to only play base GD classes as I haven’t exactly sinked thousands of hours into the game or played it the entire release cycle. However, as I’ve learned more about the skill trees like the Titan Quest trees that are added, I’ve found they are very different from the original and may be fun to mix with base classes. I still play Titan Quest sometimes, and it was clear many new skill classes were not as consistent as the official classes. 
What I’m saying is my concept of what I like may change and I’ll give any version the community wants a fair shake regardless of my personal opinion and what I wanted for this mod to begin with.
I didn’t even know I was going to write this post, was intended to be a short comment. Since I find it unlikely I could redesign such a system myself, not generally getting too involved in game stats and memorizing systems enough to visualize a system with a theme that fully fits D2, particularly with the constellations, I’m at the mercy of the community and mod designers to decide what’s best.
That I have thought this much about it, however, says something. I actually care about this mod. 
Another thought about how people play ARPG’s. I pretty much never get to endgame. I still have yet to do that even with the base Grim Dawn game. I don’t in D2 either, and often don’t even get to Hell mode. That makes me a PARTICULARLY terrible play tester. 
EDIT: I think for me, the journey is what I enjoy the most, not the end. Often times I feel I’ve realized a build enough to not keep playing, and that is my end. The gameplay and reward seems to slow significantly as I get to the endgame. So how often I play a game is generally entirely dependent upon the leveling process itself and how enjoyable it is to start at level 1. Grim Dawn actually got me to almost level 100 twice, because there’s alot more to do with a character even when high level. Still, I have alot of other things I do besides gaming, and when I stop playing a character a few months, I lose that attachment and I start over.
Starting over is always the funnest in a good ARPG (for me anyway), and the reason I’ve played so little Diablo 3 in my time.