Designated Villains: Hector, Wordsworth, and Mungo whenever they appear in the Heathcliff segments.
Ear Worm: The doo-wop style theme tune is quite insidiously catchy.
Ensemble Darkhorse: The Catillac Cats. When this show aired, it was always about Heathcliff. Only his segments were acknowledged in promos and commercial bumpers (at least on Fox Family). However, most fans of this show will agree that The Catillac Cats series was much better than Heathcliff's.
Maybe not being based on an established, but very dated comic book property had something to do with that.
Germans Love David Hasselhoff: The French got the entire show on DVD years before America because of this trope. Also, there were some toy figurines of The Catillac Cats produced in France. Even a toy Cadillac car that could be changed into a hovercraft, and trailer, just like in the show. If you're lucky enough you can find these up for auction on eBay.
It helps that the show was partially made in France.
Dixie (the athletic cat whom Mungo falls for) in the episode "The Games of Love" looks very similar to Minerva Mink, who wouldn't exist for another nine years.
The Catillac Cats episode "Time Warped", almost feels like a parody to Back to the Future Part III, what with the cats sent back in time to the Old West, the Cadillac filling in as the De Lorean, and having to use a train to get back to the present time. However the episode was made at least three years before BTTF III hit theaters.