Equality.
Equality by law is one thing. At least in the west thats largely met, bar a few exceptions like abortion rights here and there.
But a society, its values, prejudices, stereotypes, norms, expectations and many more things can't be changed as quickly as laws.
So unsurprisingly this is the hardest task for feminism, societal values reproduce themselves practically automatically and people internalize them to an insane degree, in many instances even extending to their own identity.
Questioning or changing them is obviously met with a lot of resistance.
And since entertainment media is a unique reflection of society, video games are at the center of the resulting controversy.
If this interests you in any way, there is an old but brilliant collection of essays about this. Just instead of feminism its about the changes in society due to the industrialization and instead of video games its about the reflection of this in movies.
http://www.hup.harvard.edu/catalog.php?isbn=9780674551633