A couple of posters mention choice of character names, which raises the question of players who choose “silly” names and whether this impacts on other players and their suspension of disbelief.
The reason I raise this is that my tabletop group tends to play quite serious and dark-ish fantasy, but we used to have a player who insisted on somewhat juvenile pun names for his character, which the rest of us found irritating in the role play sense.
Examples included Wassin Aname, Biggus Dickus, Reetard and similar. He left the group for other life reasons but I certainly found his name choices annoying.
Maybe I was wrong?
Yeah, I mentioned this one, so I will speak to it.
For me, joke names usually go against our first rule, which is treating other players, including the DM, with respect. Joke names often (and especially at school) indicate an attitude that the game is being treated as a joke, that the player isn't serious about engaging in a cooperative storytelling game. Alternatively, a joke or meme name might indicate that they don't really understand what a TTRPG is. Or, it might signal embarrassment, that they don't want to be seen as taking this ridiculous pastime too seriously, so are trying to salvage some cool.
In the first instance: no. My games are far from deadly serious, but if you are going to treat the game itself as a joke then this is not the right activity for you, because that attitude is deadly to the entire campaign and will wreck it for everyone else.
In the second circumstance, I am saving you from yourself, because your name is going to be an embarrassment once you understand what we are doing.
In the third case, I get it. I grew up in an era when playing D&D was a basically a scarlet letter. Still, it's not okay to cover our embarrassment by subtly throwing shade on the entire group and activity.
This is a rule that I've thought long and hard about, because it generally goes against my policy of maximizing players' freedom to run their own characters. However, I have decades of experience as a teacher of teenagers, and even more decades of experience as a DM, often of teenagers, to draw up here, and joke names just never end well.
My rule about not stealing from the party is similar in that it limits player freedom. It's not one I need at my home games, because we are all adults and if someone stole from the party, there would be good character or story reasons for it, and everyone would get it, presumably. But at a game full of teenagers, many of them beginners...disaster! Usually the group immediately implodes, everyone gets mad at the player doing it, grudges are held...it's not good. So this is a "saving them from themselves" rule until they have more life experience. We only have two hours for each session, and don't need to spend 45 minutes arguing about whether the rogue should get to steal the +1 longsword out from under the fighter and sell it (this was the incident that led to the rule).
Oh, and if, per the example above, if a teenaged player came to the table and named their character "Reetard," I would immediately shut that down and have a private word with the student about how completely and absolutely insulting and unacceptable that kind of language is. If an adult came to my table and did that, I...well, I don't know what I would do, because I would probably be flummoxed by an adult human being being such an ignorant jerk. I certainly would not play a TTRPG with them. I suspect your games were much better with that person gone. You were not wrong!