There a few complicated laws to what counts as a Mary Sue. The most notable ones are those that are already widely known such as being the centre of attention constantly, having multiple over powered traits and being liked despite what personality they may or may not have, a classic sign of Mary sue is a man/female that gets all the opposite sex attention and still manages to be the main centre of everything despite the character or player themselves possessing nothing that make said character worthy of this role. I think that most RP Mary sues are normally ego stroking tools that attempt to get everyone in the act, forcing others to be nothing more than stage props in giving this one person this perfect world.
If he liked Rainbow dash; For some reason rainbow dash would be madly in love with him, he'll be a leader of something most likely and normally the highest authority or just under it(that or free ultra hardcore outlaw that is like...a super duper sword master and is never caught or get his men killed despite poor tactics) with little actual restrictions placed, even if those lack of restriction break sensibilities or cannon lore/functionality. They rarely are capable of accepting bad things happening to them, in fact anything that just so much as nudges a living piece of furniture-other role player-in their perfect universe out of place normally send them into aggressive spirals or running away(quitting) because of this, villains or even outside forces that could debatably be considered more heroic than him/her or a simple victim never succeed and if they do manage to make an impact the great main mary sue hero will somehow be loved and admired like a patron saint for tearing that guys skull off while supposedly being the hero of the scene. In short, these mary sue normally suffer no consequence for their flaws but will allow small issues or little misadventures to take place if it keeps him interested and he can still have it all work out in the end. There is also the anti-sue. This is a character that manifest the need for attention not by adoration but for sympathy. These are like those noob character that suddenly break down for no reason and cry about getting raped just to get a scene for themselves or complain about their parents dying in fire like the billions of other orphan OC. This guy will run into tragedy no matter what...he walks down the street, gets raped. He asks for help, gets mutilated, he get poisoned and neglected while in hospital.
They tend to have grim stories and constantly scream for attention, feeling sorry for themselves and in pretty direct terms scream at other players or the audience "Woe is me. Weep for me, love me for am but a victim. The never ending victim" in some cases though, anti-sues can be created out of pure fear to be a Mary sue and vice versa so not everyone who makes these are as defias as the normal breed. But what really makes an OC Mary sue is more or less bad execution.
There are no bad ideas, just bad execution. You could be a super powerful demon warrior that sadistic. But you have to tread carefully. That the issues, most mary sue/anti-sue are made by over ambition of people still exploring their writing potential or more into feeding their ego and getting noticed than making a good story. Lets take a good mary sue:
The Doctor(doctor who) -Super intelligent. -Never ending life-or ultra long long at least-
-Somehow manage to tackle aliens and monster despite never using weapons.
-always holds the moral high ground(majority at least and thus elevating himself as hero)
-always has a chick by his arm that is young and pretty, admiration and love is typical.
-has powers to literally tear time and mess with the rules of reality to an extent.
-Also charismatic and pretty.
But here the thing: These issues are fixed by the fact that his personality bars him from abusing these traits, he doesn't use his hero status to get admiration and is more at peace just knowing people are safe. Despite solving the issue there is always some sort of backlash to it and he normally can't save everyone or get to his goal 100% like he wants to and they made it clear this effects him. They put later restrictions on his time travel and even shown that it can fail. His attempt to defeat daleks failed as they came back full force later on. His rival-the master-has totally bought him down to pathetic and harmless in stature in the past and he has lost those who trusted him most.
He stood by and ignored slave work at one point and never gave it a second thought to much later. The fact he also never uses weapons or avoids directly hurting/killing people has directly crippled alot of his powers, meaning he can only fight with none conventional means. all the doctors victories are small steps, he rarely wins for sure and as such despite the massive accomplishment he makes, he never manages to finally draw the curtain on the scenario and most of the time the sheets he has to draw are those over the coffins of the ones he loves or his hope. the final note is that the doctor stance also explains his genius to an extent as well-born in advanced society, very old but not physically degraded(explained via restoration routines and travelling the very fabrics of reality) sometimes it all comes down to just adequately explaining their powers and how they tie into each other.
You'd have a hard time saying you are a master mage and a ninja at age 21. So, yes. The doctor. Alien time lord, ender of war, destroyer of a race and immortal(mostly) genius. That the essence of a Mary sue but his development and his writing as well as the writing of the world that he is in makes it all stand up. I think that Mary sues are more of a abbreviation of general bad design approach and treatments of the scenes/development all around or people getting too big for their boots rather than terrible ideas overall. Although there are Mary sues generated by personality-rampant emotions, self indulgence-I don't believe these are the norm at least in actual media/writing but in RP it is much more common.