If I had to put it down to one thing - it's probably the inclusion of a physics engine. While I love that the in physics adds quite a bit of depth to gameplay, physics in most games always feels a bit off and cartoony - big bounces, weird launch trajectories, rag doll physics, etc. This, when combined with the slightly larger than life animation, is what gives the game it's balloon-y feel IMO.
However, I don't think the actual visuals play a huge part in making them feel different. Outside of unchangeable factors - engine feel, 2d vs 3d, super modern-day sharp graphics vs. pixelated visuals, and actual light sources, I think Larian's done a decent job of matching up the visuals. Both games can have earthy, gloomy, monochromatic in some locations, and others that are more bright and colorful.
In terms of cartoony blood and gore, I think both games are cartoony in their own ways. BG3 with their pools of blood. BG2 with their anime-esque critical-it gibs - people literally exploding into meat chunks when hit by a weapon. I suspect in both cases, it's deliberately cartoonish to avoid anything too gruesome. As BG2 did get an ESRB rating of T despite the chunking.
Lastly, BG2's UI Design (sectioning the playable world into its own little area) is basically no longer used. This is both due to updated design best practices, but the requirement to keep UIs "responsive" for various wide-screens ratios. Even the BG2 EE version has a similar updated UI to accommodate wide screens.
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