Mary-Sues are often wish-fulfillments of their creator. Think, super-rich, super good-looking, or super strong.
Yeah, sure, they could be any of these things and not be a Mary-Sue. But, these things need to have flaws/problems that outweigh them.
For example, Joe Kingman in The Game Plan is a super-rich football player who has a large ego and he's also extremely selfish. These are problems he has to overcome in order to be a better dad.
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Mary-Sue Detector
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