The first three seasons were a bit hit and miss, with every good idea undone by a number of bad ideas; but by season four and five the show had finally hit its stride as Robin, Superman and the Justice League all made appearances, helping to return the show to its more comic-book roots.
As most fans of Batman know it's the villains that really help make a Batman story good and so in The Batman the usual suspects such as the Joker, Penguin and the Riddler all appeared, with often quite radical makeovers. These guys were joined by the likes of Harley Quinn, Dr Hugo Strange and Scarface, as well as lesser known villains Everywhere Man, Clue Master, Ragdoll and Solomon Grundy. Sometimes characters didn't work so well as they had on previous animated Batman shows (Catwoman, Mr Freeze etc), but in some cases The Batman managed to provide an interesting alternative to a villain's back story. In the case of the first Clayface (there were two Clayfaces on the show in total), the character became the most interesting villain of season one and proved that there was hope for the series.
Oh and veteran DC animation actors such as Kevin Conroy, Mark Hamill, Clancy Brown and Dana Delany also turned up during the course of the show's run.