...Didn't expect to start my morning by critiquing pixel art, but alright then. ^^;
Anyway, I'd say the major differences are:
Lack of hue: with the new assets, pretty much everything but the protagonist's hair is shaded by removing value, resulting in lots of dull gray-ish colors that ultimately affect the mood of each scene. The old assets, on the other hand, shade by changing hues (for example, all the plants are shaded with navy blue and turquoise rather than 'dark green'), making the environment look vibrant, because it's literally full of color.
Lack of detail: Simply put, the new assets just aren't drawn as well. =/ Either that, or you're picking bad floor/wall textures...but seeing as you did such a good job with the old assets, I doubt it's your fault.
For example: the floor in the last image clearly looks like a floor, with simple lines that give the impression of wood planks. The wall is clearly a brick wall, with a well-defined baseboard to separate it from the floor. The contrast of the small bricks and big, almost un-defined planks is also nice.
But with the new assets, it looks like the artist just put colors in places to vaguely imply shapes, and left it at that. The normal walls are a nice gradient, but that's all they are: a gradient, that kinda clashes with the rest of the environment. The floor texture could easily be mistaken for a wall texture, as it's just a default-looking brick pattern (that doesn't even tesselate correctly...you can see spots where two bricks are layered on top of each other instead of being offset). The second floor texture is just random square tiles that don't really look like anything, while the wall texture seems to imply columns (???) but honestly just looks like something AI-generated: a vague idea without a specific form. All of this adds up, making the environments feel unimportant and creating a less immersive world.
Lack of style: the old assets clearly have a cute, rounded artstyle that affects the look of every object: notice the rounded corners on the leaves of plants, the corners of tables, the characters' bodies, even. They also have a very faint-but-noticeable blue-leaning color scheme that helps blend everything together. It's a professional level of cohesion that makes sure all the assets look like they belong together in every scene, making for an aesthetically-pleasing end product.
The new assets, though...unfortunately, they don't have quite as much of that. Besides sharing a gray-leaning color scheme (yay...) and a similar rendering style, they clearly were made to simply represent objects, rather than actually look good together. For example, the bookshelf stands out because it contains colorful books, but the distinct lack of color in most of the other assets makes it clear that that's just coincidence. If they had made the books brown instead, it would have looked even more dull, yes, but it might have hinted at an actual color style-- like they were purposely trying to make an asset pack with a somber, depressing feel. But no, they just randomly threw in some colorful books, just like they randomly throw a bright color on some of the character sprites. Randomness isn't a style, and it won't make the viewer feel something.
