These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Anvilicious – Like a lot of 80s cartoon series, a lot of this show's episodes were focused on some specific "moral", and weren't very subtle about it.
Somewhat justified in that Lion-O needed to be taught these lessons and quickly so he could become an effective leader.
Ass Pull – Some characters (including Lion-O) somehow get massive powers on convenient moments, like Cheetara's psychic Limit Break.
Complete Monster - while Mumm-Ra does come VERY close, he does have a Pet the Dog moment—even if Ma-Mutt isn't a Morality Pet, it's still an accomplishment for this setting's version of Satan. There is a definite example, though: Plutar, a one-shot villain who is among the 3 most wanted for being a sadistic, genocidal Walking Wasteland, and would have been around to be a threat for more than half an episode if not for his Weaksauce Weakness. Thankfully, he doesn't appear in another episode, implying he got executed.
Tygra also garnered some decent popularity, especially with girls watching the show. He was the Thundercat besides Lion-O to get a new figure in Bandai's line of classic ThunderCats. Many of his fans were upset by the changes made to his personality in the 2011 reboot.
Pumyra was under-utilized because the producers often didn't know just what to do with her, but many fans claim her as their favorite character. The anticipation surrounding her appearance in the 2011 reboot is insane.
Nightmare Fuel: As creepy as Mumm-Ra was, the Living Ooze was definitely the scariest thing on the show.
A nightmare-tastic episode has Lion-O was trapped in some creepy catacombs with these Morlock-esque creepers with gigantic eyes, speaking about how they had been driven out of the world by people being too terrified of what was written in their books.
Since they're supposed to be "normal" where they come from, Alien Scrappy also.
Special mention should go to Snarfer, who was just downright annoying. Also the three new ThunderCats from season 2 weren't very well-received.
Judging by the new animated trailer, they may be attempting to rescue Snarf from the Scrappy heap. They were actually able to do this by changing the Snarf race into a species that doesn't talk.
The character probably most deserving of fans' hatred is Captain Bragg, who made the Mutants and Lunatacs look like a bunch of chumps and put them on a train out of the series. He gets away with hypnotizing WilyKat and tricking him into helping him and doesn't get punished for it. Bragg doesn't even have any real coolness to him to make up for it, he's just an old circus ringleader with a talking crow.