Audience-Alienating Premise: An anime-inspired superhero action show.... starring descendants of the Looney Tunes? The show tries to combine slapstick and sight gags with tense action scenes, creating a disconnect that makes the whole thing impossible to take seriously. Even people who like the show have a hard time with this.
Broken Base: The show had many detractors when it was still airing, bemoaning it as awful. But nowadays you can find a lot of devoted fans for the show who felt that it was an interesting direction even if it differed from the usual Looney Tunes formula.
Ace is supposed to be the greatest hero of all time, but has some questionable moments, especially in the second season. In "Secrets of the Guardian Strike Sword," for example, he calls out his rival for attacking him from behind, then later in the same fight takes advantage of said rival's distraction to attack him from behind. Worse, that same fight is supposed to prove that Ace is a "true warrior" and thus the one who really deserves to wield his Cool Sword.
The Royal Tweetums. Much like ancestor Tweety in his very first 1940s incarnations, Tweetums is a self-centered jerkass who runs around getting into danger, is very obnoxious, and causes all sorts of pain and suffering for his caretakers when he's not treating them like crap. When foe Sylth Vester comes along, his reasons for trying to take out the little bastard include having had his head shoved into a singularity; he and Duck actually bond some over the abuse Tweetums has put both of them through. And the show wants you to believe installing him on the throne will prevent centuries of galactic warfare.
Ensemble Darkhorse: Rev and Tech, if only because they're the only characters with somewhat developed personalities and actually have believable and likeable interactions.
Also Rev's family (who showed up in "Family Business") got a MASSIVE fan base because of their quirky natures.
Genius Bonus: At one point, Duck calls a saber-toothed tiger "Smiley." While this might at first seem to be a jab at the... well, "Saber Teeth," the actual name for a saber-toothed tiger is "Smilodon." That's pronounced "Smile-oh-dawn," for reference.
Ho Yay: Some fans read a degree of UST into Tech and Rev's friendship. In the second season, the creators may have picked up on this and, in an attempt to reduce it, had Tech and Rev show more animosity towards each other. However, this just made them look like a bickering old married couple.
The episode "Family Business" is a goldmine for this with a lot of Does This Remind You of Anything? moments. Rev's parents react to Tech as a stereotypical parent would react to their child's boyfriend, with not trusting him and with sarcastic insults.
Rev: If you weren't a coyote - and a guy - I'd kiss you! Tech: Then lucky for me, I'm both.
In "Weathering Heights", during the fight against Weather Vane, Tech saves Rev and the following exchange happens:
Ace: (about Rev) I was going to save you next. Tech: I got impatient. (winks at Rev)
For some reason Rev always gets close to Tech at certain moments, places his hand on his shoulder and stays that way. No reason is given, he just does that sometimes.
Jerkass Woobie: Danger Duck. You can't help but to feel sorry for the crap he goes through, even if he did a Jerkass move beforehand.
Tainted by the Preview: The early character designs were extremely stylized and overly spikey, and the characterizations were written to be so X-TREEEEEME that it instantly turned people off. The show was being mocked before it even came out, and many of those people continued to mock it while refusing to even watch it. There were even people still complaining about the character designs not realizing those designs had been changed.
People who thought the idea of redesigning the Looney Tunes as action heroes had promise were put off by the show's over-reliance on Original Generation characters. The first season in particular was all original characters, with the only Looney Tunes characters present at all being the main six. Season two introduced more Looney Tunes characters like Sylvester, Elmer Fudd, and Marvin the Martian, but still tended to rely heavily on original generation characters.
People who thought the series was actually a decent action/adventure show usually argue that including the Looney Tunes at all was a mistake and would have preferred all original characters and a focus on action instead of comedy.
Took the Bad Film Seriously: As shallow as the writing often was, the voice acting is arguably one of the few saving graces of the series.
Uncertain Audience: This show's biggest problem is that after the negative backlash to the original offering, the creators seemed unwilling to commit to being either too serious or too comical for fear of alienating potential viewers even more. As a result it's usually a little too silly for the action to really work, yet at the same time it also takes itself a little too seriously for the jokes and callbacks to the source material to work as well as they should've. The usual favorite example of this kind of wishy-washy treatment is at the end of the show's run where twice big rocks fall on Danger Duck and Sylth Vester, and it's supposed to be funny and forgotten after the next cut (with neither of them supposed to have a Healing Factor to explain this). Then one minute later the villains show up and Ace and Lexi can't blast their way out of a collapsed room because suddenly rocks falling on the Loonatics is something that would kill them.
Win Back the Crowd: After the failure of Looney Tunes: Back in Action, which starred the classic versions of the characters, Warner Bros. attempted this with a hipper, more modern take in an effort to connect with a younger audience. While the show did well enough to get a second season, some of the fanbase (both younger and older) were turned off by the premise.