I always feel so conflicted about Mary Sue as a term and criticism. Because of the one hand I feel like that is a pretty recognisable type of the character and a major flaw. On the other hand it completely lost its meaning and even if we don't count obviously bad faith criticism (like calling every female character who isn't damsel in distress Mary Sue), there are people using this term left and right for completely wrong reasons.
The OG Mary Sue is overpowered character who is "super cool" and "unique", and she is better than any other character (especially canon main characters), and there is no real stakes for her, because she easily wins against every opponent.
Now people tend to call any character with unique abilities Mary Sue. Or treat any Chosen One character like that. There you get ridiculous takes like "actually, Anakin Skywalker is Mary Sue".
But here is the thing. True Mary Sue is not a question of character's abilities. It's a question of narrative. Mary Sue stories are bad (not in any moral sense of course bit from artistic point of view) because they are really flat. Main character doesn't have any actual flaws (if you remember these old OG fics, Mary Sue character always was like "oh, but I have one flaw, I can't do cross-stitch"), they super easily overcome any obstacle they come across, and overall media with Mary Sue protagonist looks like a boring game walkthrough.
Normal character can have cool unique talents and be very powerful, but they are also have flaws and get challenged that are really hard for them, and they can't just overcome these challenges with their talents and powers. Like, for example, Anakin is a powerful Force user, but it didn't help him at all. He actually failed miserably.
True Mary Sue moves through plot with barely any friction. Their wins are never earned, their fails are completely fake (you thought the character made the wrong decision? No, they were completely right all along!), author carefully protects them from anything that can be considered true weakness. They may be allowed to have some cute, sympathetic weakness as a treat, but it never should be off-putting or get character into serious trouble.
