I think some people judged it for not being BTAS instead of taking it on its own merits.
I agree. True, it paled next to B:TAS, but it wasn't awful, just relatively mediocre for the first season or two but with some interesting aspects, and improving over time.
And I think a lot of people had trouble letting go of Mark Hamill and judging TB's Joker on his own merits as a character. I think Kevin Michael Richardson did an amazingly good job as Joker -- very different from Hamill's (though there was some Hamill influence on the voice he used), but a virtuoso performance in its own right. As amazing as Hamill was in the role, Richardson has one of the most astonishingly versatile and mutable voices in the business and he was able to give Joker an extraordinary performance range. There was one episode where Batman used VR to go inside Joker's mind, manifested as a virtual Gotham whose every inhabitant was an incarnation of Joker, and each one had a different voice and personality even though Richardson was playing every one. It was amazing, and for all his talents, Hamill could never have pulled it off as well.
(And yes, in this show it was "Joker," not "the Joker." Since it was "the Batman" rather than just "Batman," I guess they wanted to maintain the contrast.)
TB also had an interestingly creepy take on the Riddler (played by Robert Englund) and made much better use of Hugo Strange (initially played by Frank Gorshin until he passed away) than B:TAS did. And the direct-to-DVD movie The Batman vs. Dracula is a fairly good film, much more intense and adult than the show was able to get.
First I just want to say, I don't hate this because its not B:TAS. I hate it because it was horribly designed and written. I might have been able to get over the designs if it was written better (except Joker, his design looks like a very bad practical joke), but it wasn't. It was medicore at best, atleast with everything i've seen. I've seen an ok amount of episodes (mostly from the first two seasons, but a bit from after that), and all I remember that was positive about the show was thinking that Batgirl wasn't a horrible character, and was definately better than Batman. The villains were all bad, I never saw an episode that had any of the villains being the least bit interesting, much less threatening. I did watch The batman vs. Dracula. It was very bad, and I remember thinking it was worse than even the normal show.
I suppose Joker from The Batman could have had a good voice actor, I don't remember his voice to well. I just looked up a video of clips of him from The Batman. The voice acting seemed decent. He still has the worst Joker design I think is possible for someone to draw and still be barely recongniseable as Joker. I also seem to recall him actually engaging in a lot of fighting and jumping around like Marvel's Batroc the Leaper (with out the french accent) but its been awhile and I could be wrong about how often he did that. Still, I've liked other versions of Joker that had nothing to do with Hamill's one (which I think is the definative Joker in any media), so it definately wasn't a bias toward B:TAS Joker that makes me hate The batman's villain they call the Joker.
Also, from looking up clips, I just saw they actually used Harley Quinn in The Batman. She looked stupid (although stupidest female design for that show goes to Catwoman, who looks hilariously idiotic) and was apparently a failed TV psycologist. It honestly surprised me that they used her at all, and the clip I saw had her being as mediocre as everyone else on the show. Harley has always been one of my favorite villains, so it sucks that she was on that show. It makes me wonder who else they messed up with that show. I guess I'll have to see if BTB using villains nobody has used because they suck is better or worse than The Batman's way of taking famous bat villain's names and powers and sticking them on characters who (both design and just writing wise) are barely recognisable as the characters they are supposed to be.