Bioshock really wanted to be an RPG, but it got caught up in being approachable. If you compare it to shooters with more robust RPG elements, like System Shock 2 or Deus Ex, its "RPGness" feels very lacking. When you come down to it, though, the only "character customization" available in Bioshock is the Little Sister choices and deciding which Plasmids to focus on. Not much different, really, than upgrading your weapon in Call of Duty. Neither is an RPG but there's still a degree of customization. Certainly, this makes it more than "just an FPS" but it doesn't necessarily mean it's an RPG. There's something undefined floating in the gap between the two.