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.
Broken Base: The hop from 2D to CGI. While some are welcome to the change in art style and are impressed by the visuals, some others would have liked to see the Animesque, Pretty Cure-styled visuals seen in the original PV.
On a related note, there are those that are annoyed at how the show was changed from a darker superhero series meant for teens and young adults into a Lighter and Softer show geared towards younger audiences, while others argue that this change will help it gain a wider audience, since networks were not interested in the series until it was toned down.
Chat Noir's early 2D incarnation, Felix, is between Base Breaker and Flame Bait status. Even as technically Felix is the same person as Adrien, his design implied enough maturity and perk to make some fans rant about him being better than Adrien.
Nathanael, one of Marinette's classmates, seems to be quite popular despite being a background character with barely a couple minutes of screentime. However, after an episode where he was transformed into a villain and revealed to have a one-sided crush on Marinette, his popularity has increased dramatically.
A few have started calling the ship Chatbug or — more straightforwardly — Catbug.
LadyNoir is also a popular term for the pairing in the fandom
The Scrappy: Chloe is easily the most loathed character on the show. Her actions include stealing someone else's design in a contest, breaking other people's possessions (while blaming everyone else), and using her father's status to get others into trouble. She is also very clingy towards Adrien, who clearly doesn't return her affections. Worst so far is how she treated Kim. After he confessed to her on Valentine's Day, she sent out a picture of him being rejected to everyone in their class for no reason other than to humiliate him. Add this to her nasty treatment of Marinette on a regular basis, and it's easy to see why she's so hated.
Visual Effects of Awesome: As noted on the main page, the CG version of Miraculous Ladybug was produced for $11.5M, more than most Western productions. This has blessed the show with highly-detailed visual design, dynamic battle choreography, expressive animation and particle effects.
The video game:
Big Lipped Alligator Moment: Getting the free game by spelling S-P-E-C-I-A-L in the arcade game treated the player to ... a human wedding?!