The entire DCAU is overated, JL and JLU in particular. I'd go on, but I really feel there's no explanation neccessary.
Instead, I'll turn to Kim Possible and W.I.T.C.H, which are moderately good shows, but waaay overrated by their fanbases.
Kim Possible really had a lot of flaws to accompony the strengths. One of the biggest was the characters of Kim and Ron themselves. The writers tried too hard to make Kim a strong female role model that she came across as arrogant and off-putting on several occasions. On the flipside, Ron became more and more of a Butt Monkey as the show went on, only regaining dignity in the last season. Will Friedell pitching his voice up did not help matters. The strengths of Kim Possible are good, but they're not good enough to overlook the flaws. :sweat:
As for W.I.T.C.H, I was vastly left unimpressed by much of the first season, especially when compared to the Meridian Arc of the original comics. Much of the plot was dragged out and boring, and there was character butchering (Cornelia) and annoying new characters (Blunk) aplenty. The second season, thanks to Greg Weisman, was the opposite; it was very good. The villain Nerissa and her plan were especially well-written. But that finale...ugh. I can't understand some folks' praise for that finale; as it consisted of just the girls going up against the boring Cedric for the umpteenth time, with the gimmick of Cedric having supah-powers! I don't care if all loose ends were tied up or not, I expect finales to be cool, and this one was lame. Even the first season's finale was better, and that's saying something. :yawn:
Oh, and concerning Avatar, I agree that it can be overrated (really, what popular thing can't be?) but I think people who underrate it because of the third season are just being silly. The third season was a drop from the second, yes, but it was still high quality nonetheless, especially the second half. It's not as if the third season was bad, it's just not as just as the second season, and really, the second season wasn't completely perfect either. Saying that the whole show went down the drain because of the final season being somewhat lesser than the previous one is ridiculous and IMHO rather shallow.