They're all brilliantly made, but the designs don't reflect the stylized originals. I don't think most of the issues people have would be raised if these were the interpretations of these factions/classes in Diablo IV - but Diablo II classes have very well defined precedents as established characters, widely available detailed concept art & high(er) resolution renders which shows that these designs don't reflect the spirit of the original, and are re-interpretations that deviate considerably.

There's a red herring of "realism" that gets thrown around when criticizing Diablo 3's visual style compared to Diablo II's (which is not, and never was - 'realistic) - when people said 'realism' compared to D3 - what they really tend to mean is dark fantasy. Grit & grime, rough decay, sharp, edgy, shadowed, uncomfortable & evocative. Diablo 3's aesthetic was not good at being a dark fantasy - even if many elements were dark fantasy, the style was brought to life in WoW-like high fantasy.

Diablo II's protagonists are meant to look like 90s comic book superheroes in a dark fantasy setting - not "realistic" They aren't supposed to look like "realistic, capable warriors", they're supposed to look like arrogant edgy comic book archetypes.

Edit: my opinion of each one is as follows:

The barbarian looks like a powerlifting dad with a day job down the steel mill (and where's his ponytail?!) - his being almost grotesque in the original was half the point of his character - the others had literal magic & the power of heaven on their side, while he has almost comically exaggerated physical prowess to keep up.

The Paladin's looks a bit shaggy & rugged when you'd normally imagine a zealous religious order being all about close-cut smooth-shaven order. He even carries around a giant blade he can shave with, so why doesn't he?!

The druid went from a pothead bike mechanic with a few dogs who likes to smoke meat - into that pothead's dad who eats most of the meat & likes a bit of ice mixed in the green from time to time. I don't have an issue with the design as it still fits the character, but it seems an unnecessary change.

The assassin's gone from an Eastern-European featured femme fatale to a Pacific-Asian, which is fine, but you might as well have appearance selectors if you're going to start race-swapping characters. Would be better to have options for all of them rather than doing it arbitrarily.

The original Amazon was an absurd depiction of sexualized power - but that was a good thing. The new one just looks like someone else who doesn't fit the character at all wearing a similar costume.

The sorceress looks mostly fine - but her original design was more waifish with a sharp look of almost academic upper class arrogance - and that seems to be missing here.

The necromancer, I honestly have zero issue with. He could've been made to look like an albino guy or someone who's hair turned unnaturally & prematurely white from his work with poisons & dark magic - but creepy old dude still works fine. You have a lot more leeway with a design like this as just having bones, black leather & white hair on a skinny sharp-featured guy with a big nose gets you most of the way there.