This would be my own position on Sues/Stus, as again inspired by a BRPS post
OC does NOT equal Sue/Stu. Canon does NOT necessarily escape it. (In fact, quite often, canon characters are egregious Sues/Stus (Prime example would be Laurel Hamilton's Anita Blake).
Sues/Stus are as they are written/played, not as they are made.
You can have an IC character with EVERY potential Sue/Stu trait (E.G., beautiful, talented, desireable, powerful, tragic background, closely related to or dating some popular canon character, hell, even color changing eyes) in the book, and still not have a Sue/Stu.
In gaming, the divide starts with how you play it. Sue/Stu players never let anyone else get a chance to solve a problem, to take the spotlight, etc. They find 1001 ways to head this off with meaningless driveling drama that contributes nothing to the plot overall, and ONLY serves to draw attention back to the Sue, and to show how SPECIAL the Sue is COMPARED to everyone else. They never lose. They tend to be the alpha and the omega of EVERYTHING. If they see others in an intense or interesting scene, and that includes a private one with long buildup, they will INTRUDE. No one else can get that spotlight!
This player, meanwhile, will expect, nay, DEMAND that that Sue/Stu be universally loved/respected, even if the character does NOTHING IC to warrant this, and, in fact, does pretty much everything to guarantee that such a reaction makes NO sense whatsoever. If being 'nice' to others with the use of those Sue/Stu powers, it will be in such a way as to insta-resolve any issues they may have, in spite of the players often not wishing such things to be resolved, as they are the entire point of PLAYING.
Sue/Stus also will randomly pull new shiny specialness out of their hind ends. Their players are often resistant to sheeting, and if forced to sheet, will be resistant to being anything but vague. Even if called on this, and forced to be specific, they will still suddenly be holding the exact right thing, for all it's not on the equipment list, or suddenly be able to use a formerly non-existant power, or use a power that formerly could not be used in such a way not as a part of natural character developement, but for gross convenience and further seld-aggrandizement.
Sue/Stu is NEVER about everyone having a good time, it's ALL about the personal wish-fulfillment of the player. Now, some personal wish-fulfillment is fine, so long as it doesn't get in the way of the OTHER PLAYERS, who want to have a good time, too. Self insertion is also pretty common, though not ALL characters based on self are again, necessarily Sue/Stu, or even necessarily self-insertion. That again goes more toward that personal wish-fullfillment line, and player ability to differentiate IC from OOC.
Sue/Stu players are generally terrible at differentiating IC from OOC.
Now, written Sue/Stu can be avoided even by the voice central to the book, such as one posed in first person singular, even a powerful and attractive one again. Step one is that that character has real flaws, and admits it. They don't always win every battle, at least NOT ALL BY THEMSELVES. They probably are not universally liked, even if quite likeable to the reader, though not everyone who DISLIKES them will automatically be bad/evil/antisocial. They might sometimes be SLIGHTLY misguided with some characters, but it could all just boil down to a matter of personal taste. NOT everyone should like everyone...even a likeable someone.
IF a character is very well liked, in either event, they must EARN it through actions and words both. Sues/Stus simply DECLARE themselves well-liked. They also declare themselves right, even when they clearly are not (and the player stands behind this, just to differentiate that from a well-played character who believes itself never wrong, but has a PLAYER who DOES NOT agree). Sues/Stus also declare themselves intelligent, yet tend not to be, at the top of various professions, yet play like amateurs, etc. Again, the PLAYER will be in full accord with the defense of the Sue/Stu position, no matter how ridiculous or destructive of the rest of the game it might be.
Of course, just to confound the issue, the VERY worst Sue/Stu players will accuse others of the traits inherent to the breed...FREQUENTLY, especially if they feel upstaged, or if the player (or even GM) has called them out for their foolishness. ANY character who is capable or well-liked is seen as a threat by this sort of player.
As for the tragic background (There are games where this is OK, and where it is not. If it is, that's up to the GM, frankly, and the sort of interactions and situations they wish to have played out, but GMs should be up front about how great a roll this can take, and some who are upset by this sort of thing should probably avoid games where it's a focal point) Sues/Stus do not play this to explore possibilities, to try to be realistic, or to try to be interesting for the sake of others, but only to have endless reasons to angst and yet again call attention to their characters.