I don't think the problem with Rey is that she's a "Mary Sue," so much, I mean she is obviously Force sensitive so she's going to be super powerful. I think the main problem is the pacing of her character's growth and lack of explanation/filling in the gaps there. Her character just feels pretty hollow and wooden. There are many times where she just suddenly seems to jump to a higher level of power with little reasoning. Luke had a much slower climb to his grasping of the Force and gaining power, and struggled more in doing so (and that was WITH the training of Obi Wan and then Yoda). The result is he feels a lot more human, which is why I think he has more of a lasting impact and many can relate to hm more than Rey, who almost seems robotic in her much quicker leap in power (not to mention relative lack of personality).
As an audience we need to be able to relate to a character by seeing them with flaws, and often undergo a long and arduous struggle to get from the bottom to the top. That's what makes the hero's journey so distinct and memorable, and why I think Star Wars was so impactful in the first place. I don't really get that sense with Rey so much. Without someone we can relate to as an audience, why bother? Why should we care about the larger story around it all?
"We hold these truths to be self-evident - All men and women created by the, go-you know - you know the thing!" - Joe Biden
