I actually disagree. I thought the more simplistic style was what led the artists down a darker path in rendering the characters. The only three members of the Rogues Gallery that I preferred in the original TAS style were the Joker, the Riddler, and the Penguin (even though after his redesign he had those awesome pet Leopard seals)... though the Joker's JL version is top notch as well in terms of design.
Other than that, the season 4 versions are my preferred versions. Freeze and Scarecrow are exactly how I would envision them. Freeze is as cold externally as his calloused heart has become since Nora left him, and the Scarecrow is little more than a walking corpse. Croc actually looks reptilian and is held in an aquarium tank while in lock-up. Bane reappears in studded leather with jagged teeth and red eyes. All outstanding... I watch TAS about every other week it seems since it was released on DVD. My only regret was that we didn't see Ra's al Ghul in season 4.