Diablo 2 is peak Blizzard, back when they were firing on all cylinders. Same with Warcraft 3. Generally, I see people praising Diablo 1 for it's darker atmosphere and better actual hell aesthetics, and then Diablo 2 for everything else.
It depends on which Diablo II you played. The game when first released was a rushed mediocre game. They didn't even finish Act IV. Years later you would have an entire new expansion, bosses, characters, items, charms, rune words, and so much new content that it more than doubled what the original had to offer. It was finally a complete game. But it took years of patching and revising the original game to get there.
Diablo 1 has superior lore, story, atmosphere. However, movement is slow and the gameplay does feel dated by today's standards. Diablo 2 packaged with the expansion, however, remained fun to play even to this day.
I watched so many hour-long lore videos; the one time I actually knew a game's lore by heart. Bought the 4 (?) books, read half of them. God it was such peak fantasy writing. Of all Blizzard games, Diablo is probably the one I've actually engaged with the most. Diablo 3 tried so many things that were considered new at the time. Shit they halfway invented battlepasses by adding seasonal objectives to an already seasonal game. The pixelated anniversary event. All the weird tie-in promo pets and shit. The pre-release campaign where you unlocked banners through the website.. Witch Doctor being a reskinned necro was so fucking kino.
Runes are probably the biggest difference - they were pretty uncommon in Lord of Destruction so getting the busted words in a normal playthrough was very unlikely even if you made a point of holding onto mid/high runes and cubing them when you could. In Resurrected they drop like candy.
My big complaint with immunities is how uneven they were: Magic resists/immunes were so rare you could work around them but lightning and especially fire immunes were so common that, like you said, many builds became borderline unusable without specialized gear. It would have made sense if Magic were rare or low-powered compared to other damage types . . . but the Necro, Paladin, and Barb all get strong Magic attacks.
Apparently some bugs still remain. Hard to find documentation for this mod, but my understanding is that there is bugged item effects from Hellfire that still don't work 30 years later. I just started the game so I don't have the context or even know if there is a way to fix these bugs, but I am having a blast regardless. Cut Content from the game is neat to look at, it was originally going to be much more ambitious and some of it can even be restored in-game(like various cut spells).
The no Battlenet support thing was a Blizzard requirement actually, if I am reading my info correctly. It would be trivial to make Hellfire online compatible, but they were told not to do that. I guess the higher ups didn't have any faith in the project.
I had a similar experience after coming back to D1+2 and playing regularly last year/this year. I'm curious if whether someone likes 1 or 2 the most is a litmus test for what people actually want out of an action/loot RPG: I prefer a lot of aspects in D1, but I admit that's because I don't play RPGs for the loot grind aspect.