I agree with a lot of these posts so far, so I'm gonna try to talk about ones that haven't been discussed much yet. I try not to judge anyone too harshly for liking certain kinds of music, but I might raise an eyebrow. Unless you're an asshole, in which case you and the music you like suck.
Kinda been touched on already, but while you have your pretentious Fantano types, you also have those people who are obsessed with
one band or genre only. When you're a teenager, I'd say this is pretty normal ( and usually has the mindset of "I will always love X band and nothing will ever top them!!"). But then you grow out of it. Those motherfuckers who go "Oh, you like that album by the band? Well, you should listen to
this album instead because it's way better and that other album sucks" or "You only like that popular song by X band? LOL you're not a real fan" are mentally stunted. I'd say this is most common in metal and rock fans.
Closely related to that one is people who wax poetic about how much they love classical music. People who only listen to classical and brag about it think they are smarter and better than everyone else. If you are one of these people, you are a pretentious fuck.
I've had the fortune of never meeting anyone who has been a fan of a tranny that makes music. However, if I do meet one, I will know to avoid him like the plague. It's impossible to separate your image from transgenderism if you're a tranny artist. So if you are into that music, I'm assuming you're liable to troon out or are a foaming-at-the-mouth "ally."
Liking Green Day, Fall Out Boy, or Panic! At the Disco is a concern. However, if you encounter someone who claims to love all three, RUN.
I'm shocked no one has brought up Queen yet. I admit, I wasn't around when they were still active. From what people have told me, though, they weren't nearly as popular back in their time as they are now. I enjoy some Queen songs myself, but a lot of these Zoomer fans are clearly into them because "OMG Freddy was so gaaaaay!" They're basic pick-me bitches who think liking Queen makes them seem cool (it doesn't).
Another one for me is Elton John. I've never met anyone in this day and age who likes Elton John, and I don't know why anyone would.
Finally, as a Queens of the Stone Age fan, there is no bigger red flag than being a Queens of the Stone Age fan. You are either a pretentious stoner who can't go a single day without getting high, a middle-aged woman who has divorced at least once and desperately clings to the hope that 49-year-old Josh Homme will fuck you (you think I'm joking, but check the comments on some QOTSA YouTube videos and you will see what I mean), or you like alt-rock and want to look cool by liking a now somewhat obscure band
(I'd put myself in this category
).