A whole chain of discourse along the modern lines of antis/proship about shipping villains. Hero-only shippers posting callouts about someone for writing an Enemies-to-lovers villain redemption story. Undersiders fic writers going "look, they're better people than most of the heroes". S9 shippers getting hate from everyone.
Also writing RPF about your hometown capes vs writing about capes from a different city being a whole THING. "Look, it's usually fine, but just don't write about Bay capes unless you're from the Bay." "Oh, it's fine to do to New York but not to you?!" "No, you're just really bad at it."
Also age discourse with the added wrinkle of people guessing that the Wards sometimes lie about their ages but no one being able to agree in which direction.
Powered Romance vs Civilian Identities Romance vs No Powers AU all being distinct genres with very different fanbases. Powered romance writers specifically often argue that it's not RPF because they're focused on the Cape, not the person. Other fans disagree.
Civilian names generally being puns based on the cape name. Usually one BNF uses a name early on and everyone else copies them because it's easier than coming up with a new one. Using a new name for a Cape with an established civilian name isn't a faux pas, exactly, but people usually avoid it without good reason.
Some people care a lot about powers being portrayed accurately and some people care a lot about powers being used in ways that further the story. Fans actually develop a tag/term for which a story is, but no one ever uses it.
Everyone who isn't an artist considers matching team costumes cringe(this is canon per Taylor when meeting the Travelers). Everyone who IS an artist has sent Accord at least one letter thanking him for forcing a team to wear better outfits.
People who are officially dating/married MOSTLY get shipped with their spouse, but there's always a contingent claiming any official relationship is just a smokescreen for whatever reason.
"[City] just got attacked by an Endbringer, everyone I don't like is dead, my faves from a different city have transferred in" is an extremely common premise.
People mostly write about the same cities. All heroes/villains are somewhat famous, but learning enough about a city's cape population to write it well is enough work that people mostly focus on the big names. The big names are NOT necessarily NYC/LA/Chicago. Fandom is at least a little hipster and would avoid the biggest cities being their sole focus. That said, Brockton Bay is only popular with the true nerds until after Leviathan, when the normies learn it exists.
Every single team gets poly sex fic. Any team with a significant lean towards mostly men or mostly women, there's fic and art excluding or genderbending the outliers.
Brockton Bay Wards are notorious for that, with people finding all sorts of reasons to ignore Vista/Shadow Stalker/Flechette when writing their underage white boy poly pile.
Travelers showing up, having weird powered sex with your faves, and then leaving town with everyone either horny or heartbroken, is 95% of the Travelers tag.
LOTS of trans!Skitter fics. Trans in either direction depending on writer. Accusations that this is just people wanting a white bad boy to ship with their fave are common. Lots of bondage fics involving spider silk.
Tattletale is a plot device rather than a character. EXCELLENT for getting your blorbos to admit their feelings for each other. "We were fighting the Undersiders and then...well, Tattletale distracted us and they got away *sheepish blush*" is a common trope in longer fics that want to keep the Undersiders around but not see the protagonists lose.
Before Tattletale was on the scene, Dragon was used for forcing similar confessions since Dragon knows everything.
Clockblocker: Tweets "Please don't ship me with my teammates. They're my friends and I've only fucked one of them, so being shipped with anyone else really makes me uncomfortable." (I don't have the original source for this joke.) Half the fans treat this as a serious request, the other half double down on including him since he opened the door.
Everyone assumed Skitter and Grue were like 20 early on because obviously the team leaders are going to be adults even if Regent/Bitch/Tattletale are younger. When Taylor was outed a LOT of people had to do some soul searching about their previous fics. Grue remained "probably 20+ in people's minds" even afterwards.
Legend/Alexandria/Eidolon are all basically unfuckable in people's minds and no one knows why. There's more fics about Endbringer sex than any of those three.