I think stylistically the new artwork is fine. The problem is that it's not supposed to be just art. It's supposed to be functional in identifying different characters even when they appear in very small icons, condensed from their fuller expression. The shadowy style obscures differences between characters that were previously obvious, creating more confusion.
Honestly, there should be more effort to create easily noticed distinctions when working on art like this. People are already going to identify toons incorrectly just because the pictures are new. To add heavy shadows or otherwise obscure details only makes the problem worse. And not only for older players who are used to a certain look, but also to new players who simply lack the encyclopedic memory of character details that someone playing the game for multiple years can call on to ID different toons.
I don't bring this up now because I have any hope it will change in response to these comments. I only raise the issue in the hopes that the next time they decide to revamp all the artwork they consider functionality.