Well if you've grabbed a look at some of the short little video clips of the game, you'll notice the atmosphere is spot on. And judging by the looks of it, the game has some even crazier lighting and textures. I have a feeling the multiplayer aspect of the game is going to take the use of cool weaponry and plasmid usage to the next level, and not focus primarily on atmosphere.
EDIT: To add on to the co-op scenario, I feel co-op would ruin the single player atmosphere. Although you are no longer this lone sane(or so you think) man in a world full of crazy stuff, I think having a player character tagging along with you would ruin the individuality of the player's role in the story. Even in BS2, the individuality exists; you are a lone rouge big daddy, and you have the superior intellect and reasoning of being the first one created.