So one of the main features of a Mary/Marty Sue is that everyone seems to really like them and be invested in them immediately. Rey has that (Finn latches on to her quickly, Han starts treating her as a surrogate child very quickly, Leia hugs her instead of Chewie, even the antagonist Kylo seems to have an interest in her, etc.) but Luke doesn't have that.
Leia seems to think he's a bit useless at times during the escape, Han thinks he's a backwater rube, but these characters grow close over the course of the story.
I admit that a lot of people do leave off the "is treated as super important and great by everyone immediately" bit when they define a Sue a lot, but I think it is a significant part of the definition and I also think it is a big part of what people don't like even if they often fail to articulate it. There's a "look at how cool and perfect our character is!" feeling you get when sequels introduce new characters into an existing setting and all the old characters fawn over them immediately that just isn't fun to watch.
Lastly characterizing Luke as "immediately using the Force" is bullshit I am sick of seeing. Firstly, we do see him practicing and failing at it on the trip to the Death Star, secondly he only uses it in incredibly vague terms to blow up the Death Star in a way that is way closer to "having faith" than "using a super power" in the context of the story. Luke sees Obi-Wan do a mind trick in the first half of New Hope and we don't see Luke even attempt it until Return of the Jedi. The first time we see Luke use a force ability outside of the Death Star run, which again in terms of how it is presented isn't really the same as other times the force is used, is to move his light sabre on Hoth. This is months after New Hope and he still really struggles to do it. But even if I cede the blowing up of the Death Star as Luke using the force instead of trusting in the force, we still see him actually practice trying to still his mind and use it before it happens, which isn't something we get for Rey.