The tendency to assume a character that rubs you the wrong way is a Mary Sue. Also, details needing to be explicitly explained, exercise some critical thinking and you can discover some interesting things, or uncover your own bias.

For example, I have made a character who one of their listed fears was essentially, "Fear of deep water" and also had a fear of spiders. The rp this character was made for was basically a space Mecha RP [@pugbutter] was actually a part of -not saying his annoyance was referring to my character, but it did make me want to address this- Now, being largely based in space, the likelihood of this character encountering a large body of water was probably slim. However, despite it not being relevant to the RP, unless they were to land on Earth, it was rather relevant to the character. The character in question had grown up on the moon Titan, where the closest thing to a lake, let alone a sea/ocean, are pools of liquid methane, at temperatures close to -200 Celsius. I assumed this would likely lead to a natural fear of these pools, since even slipping into one of these pools would cause sever cold burns, and possibly death. Extending that idea, I assumed that having no experience with actual lakes or oceans, the idea of water as far as the eye can see, even if he has never personally experienced these things himself, combined with the existence of bizarre lifeforms - again for someone who has no actual experience with these things -  swimming all about him, would likely fill him with dread. So it would be easy to say that this character really has a fear of things he doesn't understand, or a fear of the unknown.

It's so easy to simply label a character that you don't like to be a Mary Sue, and even easier when you toss away the actual definition of a Mary Sue, and substitute your own. It makes it so that any character that you don't like, whether it be that you think they are too attractive, too smart, or that everybody likes them - which you can hardly fault a single person for in the case of roleplay - And its even easier when you read a simple detail and make your own explanation: "Oh so this is just a gender-bent version of Starfire, oh so this character is loyal too a fault, big deal, wheres the actual fault in that etc etc." It makes it so that you can dislike any character whatsoever, and instead of admitting that you just don't like the character (I personally hated Tanis Half-Elven, doesn't mean he was a bad character) you can cover up the personal bias you hold against them behind some overused criticism like 'Mary Sue/Gary Stu'

Regarding the need to explain things in explicit detail, I'll use the same RP, in which one character I once jokingly described as: A former child soldier, unapologetic, and apparently untraumatized. Potentially Psychotic... At first glance this likely would sound as if I was criticizing the fact that despite clearly having lived a harsh life, the character appeared to be entirely emotionally unaffected by their situation. Reading between the lines (And perhaps I was giving the Rper to much credit) it was rather clear that the character had adopted a psychopathic/sociopathic personality, combined with apparently copious levels of drug use, to dull their pain and hide behind a mask.

On a more minor note, the tendency for people to assume that everyone but them is somehow the norm, and that they alone are special. Everybody is special, (AWWW THATS ADOWABLE) and I'm willing to bet everyone has at least one IRL story that would make the rest of us go "C'mon dude that didn't happen. Don't lie."