Disney

I was disappointed how many key lines were changed for no reason, including some that tied in with following lines. e.g. when Ursula suggests becoming human, the new one has "Is that possible?" Where original had "Can you do that?" And both are followed by "My dear, that's what I do. It's what I live for." The original line fits the response better, and there's no reason I can think of to change it, unlike some of the other changes (like in Kiss the Girl, which I understand).

I also think the change to the opening scene (and related follow-ups) to be more dark and violent were unnecessary. In the original, you could argue that there was a lot of misunderstanding, that they didn't realize what they were really doing, while in the new one they are straight to enemies and both sides are deliberately being evil. That changed a lot of the tone of the movie in a way I didn't like.

Similarly, in Ursula's lair, Ariel was grabbed by monsters in the new version, while it was captured merfolk in the original. Major change in tone, and I liked the original in that regard.

And I really didn't like how dark and hard to see everything was in the new version. Even the dark scenes like the ship and Ursula's lair were much easier to see in the original movie.