Luke isn't a mary sue though. He was on tatooine where the only thing he could do was fly and he trained to enroll in the imperial academy as a pilot. Flight was the only thing he was proficient in in ANH. In ESB, 2 years have past so he's had time to hone his skills and practice the routines Obi-wan Showed him. He then goes to train for an undisclosed amount of time with Yoda before fighting Vader, who goes easy on him, but still defeats him. Then, he trains himself while preparing to rescue Han and still is adequate at best compared to most jedi seen in the prequels.

Mary Sue's been around for a while, since fan fics have been a thing.

http://tvtropes.org/pmwiki/pmwiki.php/Main/MarySue

I'll take two parts from the TV Tropes link there:

"She's exceptionally talented in an implausibly wide variety of areas, and may possess skills that are rare or nonexistent in the canon setting. She also lacks any realistic, or at least story-relevant, character flaws -- either that or her "flaws" are obviously meant to be endearing.

She has an unusual and dramatic Back Story. The canon protagonists are all overwhelmed with admiration for her beauty, wit, courage and other virtues, and are quick to adopt her as one of their True Companions, even characters who are usually antisocial and untrusting; if any character doesn't love her, that character gets an extremely unsympathetic portrayal."

Both paragraphs can apply to Rey in one way or another, especially the latter part of the second. It isn't a female exclusive usage either since the male version is Marty/Gary Stu which follows the same basis.

Now, the first movie can be forgiving since it serves as a introduction. Luke can be seen as a whiny brat in the first movie but grew into someone who can handle handle himself and is worth the respect. For Rey we didn't get that character development. We didn't get a movie with a proper time skip to show change. Couple in the fact that the writing for the newer movies isn't all the great and you have a problem.