Forbidden banished exiles. Especially in MMORPGs, I've found that the most common overall in-setting guild theme is "we don't belong in the society that this setting established." It's not just in MMORPGs, of course. It's often a crutch (or training wheels, if I'm being generous) for some roleplayers to participate by not participating. I'm not saying that a character (or a player for that matter) has to bend the knee to Lord So-And-So, but it's kind of hard to find plot excuses for the the forbidden banished exile to even be in the same picture, let alone get the quest to do the thing.
Plot armor dependents. Yes, it's quite likely that if I'm running the game and telling the story that I won't have the guards kill a player character on sight even if they do something exceptionally stupid just to see what happens, like stabbing a random citizen. Those kind of players don't tend to last long if their only contribution is "try to knock down the props just to see what happens on set."
People that directly lift an established character in well known fiction, often just changing the spelling, and often not even trying to resemble or act like that character. In an older MUD, I recall a "Frrodoh Bhaginz" that was a Half-Ogre hunter. Again, it didn't ruin the game, but it was annoying. I admit it can sometimes be a red flag warning, and a helpful one, if the lifted name is from chud fiction, like if they have "Rahl" in their name. It can show me what to expect, either in a tabletop group or in a MMORPG guild.
Spotlight hoggers, aka main character syndrome. Many people do it at least a little, and its pretty forgivable in small doses. Similarly, crosstalk, when players talk over each other. Which, I get, especially online, when you're excited and want to talk about 'thing', but please please please let the quieter or less social players also say what they want to say. Often some passive sexism to this one too, considering men often are socialized that their opinion is Very Important, and women are socialized to just roll their eyes and let men say their Important Thoughts.
Had a player that was part of the friend group for years, that always had to be involved in everything. Though I've finally cut him out of my life, for more IRL reasons. Which, might be justifiable in some DnD type games, but the group did a lot of WoD, with large groups, and players splitting off into sidegroups. Like, dude, just focus on your character and their faction, don't metagame to be involved in everything.
Worst was in DnD though, before I switched to pathfinder. I try hard to make sure that, in each plot arc I run, each player gets at least one 'spotlight' moment. For this one, tumultuous city of guilds, more-or-less ran by a mask council of priests, wearing masks of their gods. A player was a cleric of Thor, and she, the player, was a little more timid, and as such got talked over a bit. But this was obviously a subplot she was directly involved in, as the council was corrupt, especially the Weasley priest of Thor. She got to do a cool power-up moment, and smite him in the council chambers, then some assassins stuff happened, involving the rest of the players rushing into the now broken council chambers. Except problem player just had to know what was going on in there, and made a bit fuss planning on how to smuggle his raven familiar into the camber with her, to 'make sure everything's okay'. And like, its a pretty minor thing, but when its constant, and involving game time and rules adjudication, so his 'super smart wizard' can keep looped in on every player's stuff, its frustrating, especially since this city has his family in it, and he gets his own important plot stuff in the city!
Sometimes a group can be indecisive and flakey until/unless someone "main characters" which I do sometimes encourage within reason. If no one wants to step up and say "yes, we will cross the river" it can help prevent long awkward pauses.
Crosstalk sucks though and I only allow it if it's a crosstalky moment, like a loud social gathering where the NPCs may also crosstalk them too.
I totally agree with the spotlight focus points. I do the same thing so that more shy or inexperienced players can get an uninterrupted moment, or even episode. It works great.