FPS: Okay. I'm running down the corridor. Enemies! Circle strafe! Attack! Dodge! What's that noise? Oh - I've set off an alarm. More monsters are coming. Erm... Circle Strafe! Attack! Dodge! Oh dear - I appear to be dead. I'll have to reload.
RPG: Oh no you don't. My turn. Hmm... there's a monster round the corner. I'm not going to be able to fight him. Let's back off... ah. A machine which will resurrect me if I die. I'll activate that.
Now here's a security machine... quickly hack that fellow disarming the alarms. And by using my research skill on an organ I found in a dead body, I'm aware that anti-personnel rounds perform best against this chap. Time to go fight. He's coming at me... I point my gun towards him. I press fire. He's dead. And no-one else is coming because the security's off. In a very real way, I believe I win.
FPS: As if that proves anything...
RPG: Oh, I've had enough of your whining. Would you have been happier if I'd hacked into an enemy turret and then lured them towards a killing zone? Or just avoided him entirely by using an invisibility psionic power? Or went somewhere else, exploring for a continuing thread of the epic plot? Would you?
FPS: You finished?
RPG: Nope. But you can have a go now.
FPS: Thank you. SS2 has a gun sticking out from the bottom of the screen.
RPG: You're going to have a gun sticking out of your backside if you don't stop being facetious.
FPS: Doesn't matter. I win. Everyone knows that RPGs have to have kobolds and elves. (Thwack!)
RPG: (Recovering from delivering a sterling head-butt) Bastard.

Ultimately, System Shock 2 is a member of the Secret History of the RPG: The Action-RPG (think Dungeon Master, Eye of the Beholder, Ultima Underworld). People are going to quibble about what ratio of RPG to action it possesses (I'd say 70:30 towards RPG), but in many ways that's academic.

In the final analysis, what matters is that System Shock 2 is 93% perfect. Love it.