When Apple changes designs, they tend to be improved over the previous version and the previous version looks older by comparison. I think this is true looking at this screenshot:
This isn't quite so apparent with lighter backgrounds:
The big problem with the light backgrounds is that there's no drop shadows so it lacks depth. Darker backgrounds act as a shadow layer for lighter icons.
The colors are more vibrant in the new design, see the notes and messages icons for example. There's more contrast between all the shapes. Some of the new icons look bad e.g reminders, game center and settings but overall, it's a fresh design and very modern.
Every design will appeal more to some people than others. They are issuing one design for over 100 million people so some will be happier with it than others. Similarly, some people were tired of the old design.