Honestly for me it's a rude player above all else. Players who just don't know how to do something and mess up can be helped and I mentally file more under "inexperienced" than bad, since they can always learn.
Main point of FFXIV as a game, to me, is to have fun. Even if you're dealing with a frustrating challenge, it should be fun to keep trying on some level. When players are actively rude, whether by being nasty or deeply inconsiderate (ex. ditching newbies who are watching a story-based cutscene for the first time even when they mention that's what's going on or ask for patience), to me that compromises the ability of other people to have fun while playing. More than that, it ruins fun to a degree that is way worse than just needing to be patient or dealing with some inconvenience.
It's like getting smacked in the face by someone you thought was friendly. Douses cold water on everything.
Edit: Aaaand I just saw previous pages debates lol. XD Honestly it seems like "bad player" might be semantically too broad for the different stuff people focus on. There's skill focus and there's overall experience focus as two different aspects of playing I think. My sense is people might be putting different levels of emphasis on skill versus overall experience. This seems almost like there's an unspoken question of "is it worse to be unskilled or to be mean" in a way, which people do seem to disagree on.
As a 2 cents thing... skill focus, for me, is more of a real life point. It's something I apply really hardcore to my work, which I love, but which remains on a completely different level compared to gaming. I like to be skilled in games, but it's not why I personally play. It's important to git gud and all haha, but when I see people get really serious about it to the point that they get honestly mad about lack of skill it feels like they're missing the point of a game and lack perspective. Games are literally there to make you happy. They don't get you a better paycheck, a better house. They can sometimes lead to real life friendships or relationships but that doesn't happen because of your skill level.