The more I think about it, the more I realize my issue is mostly the character art.
I'm on board with monsters like this:
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Neat environments like this:
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But then the adventure party looks like:
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And I'm immediately turned off. There is just nothing in there that screams deadly dungeon delver. They look like they'd last about a minute in the Tomb of Horrors...
This is a great example, IMO, because the question then becomes 'why?' I do not dispute that this art (particularly the bottom one) looks different than most of 1E, 2E, or much of basic-classic's art*, but it's unclear how these characters are coded as less-prepared for the harshness of dungeon-crawling than the TSR-era adventurers (who were consistently drawn screwing up, about to die, etc.).
*mostly in that it is clearly originally meant as full-color painted or digitally-painted art, rather than line art or paintings mad by people who spent much of their time doing monotone line art. That, I think is the major difference from old and new -- bitd most of the game artists were predominantly aspiring illustrators, while modern ones are aspiring painters.
Looking at this bottom picture, I see:
A castor - likely a wizard or sorcerer, with symbols of being a knowledge-worker. Okay, glasses are anachronistic to the faux-medieval milieu D&D vaguely approaches, but it's effectively just doing the same 'communicates smart guy in party' schtick that robes with moons and stars and a pointy hat did bitd.
A lightly-to-moderately armored character -- possibly rogue, ranger, or dex-based fighter. Okay, the armor isn't historically accurate (but then again, AD&D had ring mail and studded leather and not-obviously-cuir-boilli-leather armor and such...), but assuming the things with metal edges are metal all the way through just with leather or cloth covering, it isn't worse than many IRL partial armors*
*assuming that there is an unseen helmet that is simply doffed for the portrait. That's a an obvious issue that exists with fantasy depictions from every era and every medium (so much so that even pointing it out is hackneyed at this point, but I'm doing so to acknowledge I am aware of it).
Third character is a moderate-to-heavily armored character -- fighter, paladin, cleric, who knows? Other than the helmet issue again, there's nothing specifically weak or powerful being communicated. They are a race one wouldn't have played in AD&D (although I played a Hsiao in BECMI that would be kinda along the same vein), but again not specifically better off or worse than PC races of the era (with a nifty answer to pit traps).
So what is it that makes this art speak to you differently?