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 episodes made without Van Partible. Some liked the larger cast of central characters and more accessible humour, others hated the Flanderization, increased amounts of slapstick, and Art Shift. Pretty much the only things fans agree on was that Suzy's redesign looked hideous.
The celebrity appearances can be this as well. Much like the celebrity cameos on The Simpsons, they're either welcome cameos, painfully unfunny — and in the case of certain celebrities like Donny Osmond, severely dated.
Creator's Pet: Donnie Osmond. Van Partible seemed awfully fond of using his character, especially in the much maligned Season 4, despite Donnie being an example of a severely dated, unfunny celebrity cameo, as seen in Base Breaker above. Even Johnny can't stand him. This becomes worse when you realize that Donnie's roles in those episodes (particularly the parts where he acts as the friend that Johnny does not like) can easily be portrayed a lot better by Carl.
Crowning Music of Awesome: In one episode, where a little Mexican village enlists Johnny to save them from terror, we get a little musical interlude as Johnny is brought into the village on the back of a donkey, while a harmonica and guitar rendition of Peter and the Wolf plays.
"Funny Aneurysm" Moment: The episode "Johnny Meets Farrah Fawcett," which centered on Johnny trying to get a date with Farrah before she leaves forever. Prior to 2009, this would have been a joke about how celebrity cameo characters only appear once in a series and are never seen or heard from again (unless they're so popular that they have a recurring role, as seen with James Woods and Adam West on Family Guy or any character voiced by Albert Brooks on The Simpsons). Thanks to Farrah Fawcett's death in 2009, the joke is a lot more tear-jerking.
A harsh one regarding the Hungarian dub of the show. The voice actor for Johnny Bravo (who, if you haven't heard, is an Elvis Presley-esque sexist trying to score with women) got arrested in 2012 for sexual violence charges.
In "Endless Bummer," the reflection Johnny's tanning mirror blinds a captain of an oil tanker which crashed into a rock, spilling oil into the ocean. Not so funny thanks to that BP oil spill in April 2010.
Ho Yay: Johnny landing on The Island of Beautiful Men after getting rejected from an island of angry Amazons.
Also much of it from Carl to Johnny. Carl pounces Johnny constantly (as in "Johnny Goes to Camp"), is very clingy despite Johnny shoving him away, all-too into playing Johnny's pretend date ("Charm School Johnny") or wife ("Chain Gang Johnny"), is impervious to the wiles of chicks, often thinking they're evil ("Forest Chump"), and even said he loved Johnny while standing up for him against an angry mob in "Lodge Brother Johnny". Heck, the episode "Carl Be Not Proud" is practically nothing BUT Ho Yay. Great lines like "If I can't make direct eye contact with Johnny, I'll die!" help too.
Occasionally Johnny will even do something Ho Yay-ish, like in "El Bravo Magnifico" he disguised himself as a woman and a Mexican man named "Caliente" hit on him, saying, "Surely a girl as pretty as you has kissed a man before..." Johnny's response? To break character completely and shout, "What? I was young! I-It was New Year's Eve, the cherry cola made me giddy!" He also will say things that probably aren't meant to sound weird, but do, like "Well, eatin' Carl did give me a hungry man's appetite." The episode "Endless Bummer" also includes lip contact between him and Carl (though this one was unintentional on Johnny's part). Not to mention the instances concerning background characters; so yeah, surprisingly a lot of Ho Yay for a show about a guy trying to score with women.
In an episode where Johnny wakes up in the future, roaches coexist with humans in society. "Roach" has been used as a derogatory term for a Hispanic person (particularly a Mexican, Cuban, or Puerto Rican person), and with Hispanic populations growing, some audiences who aren't familiar with the show can construe this as racist.