I don't know what it actually is but this movie is lacking of something. Maybe it would've needed a little bit more substance for the characters to evolve in but I'm really not sure if that is the missing part I'm unsuccessfully looking for. I liked the setting and the overall atmosphere a lot. Especially the combination of neon lights and the typical 80s music fitted quite well in my opinion. I also liked Theron and McAvoy and think they did a good job portraying their roles. The action sequences are nice to watch and for the most part very well done but still something holds me back to give this movie a higher rating. I found it a little hard to bond with the characters. Maybe that's why I couldn't enjoy the movie as much as I wanted to but that's not the fault of the actors more so of the script I think.
So I would say it's a movie you can watch if your looking for a easy-to-watch-nice-to-look-at movie with some good action and a badass female main character.