I'm presuming it's either about Rey being a female lead or Finn being a black stormtrooper, but I'm presuming it's the former, since most won't publicly still admit the latter as it's so clearly racist.
There is a fringe of the Star Wars fanbase that has a great deal of resentment toward a female lead in the film. They say, "It's not that she is a girl, it's that they didn't have a reason to make her one - it was just to pander" or some other such drivel. They don't realize what they are saying is that all characters by default need to be male, unless there is a really good reason (in a movie that isn't mine, because my Star Wars is for boys!).