Especially when it comes to vibrancy of the colors. They just look more saturated and less pale/dull in the earlier shows than later on.

But mostly the designs themselves are better. Also taller and more realistically proportioned. Superman in particular looks short and stocky in JL/JLU than in TNBA/STAS. Plus the change in color for Batman was a dumb choice. The dark greys and blacks would contrast amazingly with the bright colors of the rest of the League. Just look at Bruce Timm’s original designs for the League with the TNBA design, Hal, Barry, and orange shirt Aquaman. Plus the New 52 did it as well with dark grey and black against a myriad of colors. And Luthor lost his tan from show to show which was definitely weird. Plus his head looks more angular and less rounded like he went on a diet. Not to mention he lost that suave, cool persona that he projected once other mainline villains started coming around that aren’t Joker or STAS villains.

Joker’s redesign is perfect though. 100%.