The lighting in the old one, while indeed containing lots of color, was a bit blurrier and a bit more... "Muddy" I've heard it described.
This can pretty much be chalked up to the fact that her original splash is already more than 3 years old and somewhat low-res compared to the art Riot puts out today. Nothing that couldn't be fixed by going over the old one and cleaning it up a bit.
The new splash on the other hand has crystal clear lighting, and despite being just white, it feels more natural.
You call it "natural", I call it "bland and boring". The lighting in the original was awesome because of the contrast it created - Fiora with black and silver armor standing in front of the golden window looked incredible. Now it's what? Fiora in a bland white shirt standing in front of a bland white window.
As for the background as a whole; It may not be entirely to your taste, and that's okay. Nothing wrong with opinions. But you cannot deny that it's drawn well.
I have no idea why they gratuitously added all the architecture to the picture. The old splash was great because it completely focused on Fiora, standing in front of the light in shining armor. The stair and the bannner in the new one are decently drawn, but they have no business being in that picture as far as I am concerned. All they do is draw focus away from what the splash is supposed to show.
And Fiora's hair... While I'm not a particularly big fan of the cut or the pink streaks instead of red, I prefer the new splash's way of drawing it. The old one seems a bit... Messy. Fiora is a noblewoman, so I kinda expect her hair to be neat, despite her being a duelist.
Her hair isn't messy, she's supposed to be in motion, jumping forward or spinning around. You know, things a duelist does. It's actually one of the few instances where Riot's "action pose" obsession didn't end up looking forced or completely out of place. The new splash goes for the exact same thing btw - it's just hard to see because the hair is so badly drawn. It doesn't even look like hair, it looks like a single piece of plastic, drawn with two brush strokes, that she's wearing as a hat. Meanwhile, you can almost count the individual hair in the old one. It looks so much better it's not even funny.
The head in general is by far the worst part of the splash. Not only looks her hair cheaply drawn, her face can only be described as a mess. Her original face looks actually natural - the new one makes me think of a barbie puppet with too much makeup, stylised to the point of grotesque.
I'd love to hear your opinion on what exactly makes the new splash so awful in your opinion though. I've just found it a bit hard to pinpoint exactly what you dislike about the new splash (Besides Fiora's chin and cheek problems.)
When looking at both pictures, the impression I get from the new one is pretty much that an artist wanted to recreate the old one, but completely failed to see why it works as a picture. So he just took every element of the old one and threw it together in a new picture that is worse in pretty much every aspect apart from those that can be chalked up to the age of the original. The new splash simply feels like a cheap knockoff of the old one.
Also, everything I mentioned above.