D2 had a very distinct aesthetic from D3, I would suggest. I say this liking both games.
D2 has a sort of how to put… atrocity aesthetic. It’s somewhat disguised by the pixelated art and so on, but there’s a lot of outright horror in D2, and it really kind of peaks in Act III and IV, but particularly act 3 with literal piles of bodies all over the place. Even in Act I and II there’s a lot of scary, unpleasant imagery softened by the pixelated art.
You kill stuff and there’s frequently blood and body-parts everywhere in a way that’s slightly alarming.
Plus the general visual tone is slightly gothic, in terms of colours used and so on. There are also the neon greens and blues and so on of primitive lighting schemes and cheap re-colouring, but the general palette is quite distinctive. The aesthetic is also quite an intense one generally.
D3 was very different. This provoked an overreaction for sure, but really, it is different. The colour palette of D3 is totally different, and features, in particular, very heavy use of coloured lighting - not dynamic lights from spells or the like, but “this section is lit in blue, this section is lit in green” and so on. It also uses lighting somewhat softly, and has a generally less intense, more open, visual look - there’s a lot more “open space” generally, which is actually kind of unhelpful much of the time. The amount of horror/atrocity is also significantly toned down and fantasy-ized. What I think really offended people was that it moved quite close to WoW’s aesthetic, which is a decent aesthetic, but one very different to D2. WoW is essentially a derivation of both '90s macho comics - particularly Image, Rob Liefeld, Joe Madueriera etc., and really, Disney. It’s highly stylized, very strong use of colour, and quite different from the sort of more '80s gothic fantasy D2 seemed to mostly be aiming for (again, technology factors in too).
If you want to see what a modern D2 would look like, it’s very easy - Path of Exile really nails that aesthetic (albeit with a New Zealand twist). You can directly compare some areas of PoE to areas of D2 and see the relationship. That’s not really the case with D3, except for the stuff added post-RoS.
So yeah D1/D2 definitely were darker than D3 visually, more intense, less colourful, more gothic, but yes there was also an overreaction - on both sides. As well as fans being silly, devs got pretty silly because frankly there was an element of truth to fan weeping, and it touched a nerve with the devs. I don’t think it was remotely an accident, either, that RoS, the expansion for D3, was set in basically “Gothic Town”. It’s still the D3 aesthetic, but there’s much more of a sense of the gothic-ness of D2 to proceedings. In the free quasi-expansion that came later, this trend is continued and even built on - the lands that added are almost monochromatic, and feature more horror-ish badguys even than the demons and ghosts and so on of D3 and RoS.
Overall D3’s history, I think, it’s fair to argue is “The fans wildly overreacted, but they were basically right”. I mean, the reaction to the RMAH/AH and itemization of release-D3 was… not positive from D2 fans. But what happened a few years later? They ditched the RMAH and AH, re-did itemization in multiple ways and suddenly it was a game people actually liked and respected. That wasn’t just down to fans - it was down to the fact that what the fans had wanted was actually a superior design.
I would argue the aesthetic changes as shown by RoS and the post-RoS content also suggested the fan reaction to that was, again, overblown, but had a core of truth to it. I think Blizzard themselves now want MORE aesthetic separation for the Diablo universe from the WoW one, not less.
It will be very interesting to see what Blizzard do next with Diablo. They’ve been hiring people for what appears to be a major Diablo-related project, and it seems very unlikely to be a mere expansion for D3. I presume it will be D4, and I think the odds are extremely good it will be some kind of semi-persistent open-world version of Diablo (still isometric, still non-MMO, etc. of course), which learns heavily from what worked and what didn’t in both D3 and Path of Exile. Essentially Path of Exile stole a huge amount of audience/money from D3, by listening to fans, by building from D2, not starting over, and whilst that means it has some anti-accessible elements, I think there’s a lot a D4 could nick from it (not least that complexity and randomness are not bad things - that the odd shocking surprise or turn of luck or the like can be a good element for this kind of game).
