It defies explanation...
The game makes you question the very nature of the fighting itself. As I'm unloading a clip from my Tommy gun into a crazed splicer, after setting him on fire with the plasmid "incinerate," and he's screaming bizarre commands as if he believes he is still a doctor and I'm a patient, I think about how this used to be a human being trained to save lives. After years of medical school and working in hospitals on the surface, after coming to the city of Rapture hoping for a better life and a better future for him and his possible family, he is now a monster being annihilated by rounds from my machine gun. As I'm watching in awe the violent, unpredictable situations unfolding before me, sometimes so violent and chaotic it's comical, something registers close to a pang of guilt. It's them or me of course, and 99% of the time all video games boil down to killing or destroying things, so it's nothing new... But it's all really the opposite of what the people of Rapture wanted when they came to what they believed to be a Utopia. The player is made to feel a sense that with or without their presence there is a persistent world around them. The Little Sisters wander the corridors harvesting the Adam under the protection of the Big Daddys, and the Splicers scavenge for whatever they can find like mutated junkees, sometimes even fighting each other. The city is imploding both as a society and structurally, and the player is only there to witness it. Although not by coincidence, of course...
The visuals have a lot to do with why the game never really feels like a shooter. It's a setting that really puts the player into an alternate version of 1960. The player wants to see all there is to see, even if the walls are spattered with blood and there are signs everywhere of what kind of evil and cruelty man is capable of. The combat is almost secondary to the experience. Bioshock has one of the best audio experiences I have ever heard. Every little action and aspect of the environment has it's own sound. Although a lot of lines are repeated by the enemies it still gives them a sense of character and individuality. The Little Sisters and the Big Daddys are especially memorable for their look and sound. Even after playing through the game once and getting used to being revived in the Vitachambers I still want to cringe whenever being charged full force by a Big Daddy. The experience of rescuing a Little Sister is rewarding even on an emotional level, like putting a sense of right and order back into the madness. The sound of their footsteps as they run to safety is unforgettable. From my experience the fighting and gameplay are all about doing whatever works at the time, or whatever you feel like. The player is never forced into one course of action for defeating enemies or getting past obstacles. It's a lot of the reason why the game world feels so real and persistent. My first playthrough I found myself relying on my guns and obsessing over which plasmids and tonics to equip. But the second time around it has been even more entertaining to just create complete chaos whenever possible and have fun with all the abilities and combat situations. I believe it is true when the creators stated that playing Bioshock will never quite be the same experience every time for every person, and that deserves a lot of respect. Bioshock has story and characters. It is visually impressive, especially from an artistic standpoint. It has extremely high quality sound effects and dialogue. It has ultraviolence, with situations occurring that will both make you stare at the screen and laugh out loud. It's not a short game, if you take the time to explore a little. What else could anyone want?