I finally manned up and finished System Shock 2 today. For reference, the last playthrough I attempted ended on Deck 1 (you start on Deck 2), around the cargo bays. The funny thing is, it's not actually tough at all, at least on Normal: almost everything up to Deck 4 (aside from the odd security bot and those goddamn protocol droids) can be wrenched, and by the time you hit Deck 4 you've got powered armor and lots of weapons and ammo and implants and health and cyber modules and maintenance tools and whatever else, so really not much can get in your way. It's just absolutely scary as fuck. The atmosphere in the game is downright creepy, and the game goes for more of a general "you're never really safe" kind of horror (courtesy of enemies respawning when you aren't looking), rather than relying on the "OOGA BOOGA BOOGA!!" type of horror, and it works. Goddamn does it work.
Somehow the game, with its much older graphics engine, manages to be scarier than Bioshock and its high-def bump-mapped realisticly glistening enemies, despite both games having a similar approach to creepiness and horror. At first I thought it was because Bioshock gets retardedly easy, even on Hard, once you start racking up all the good upgrades, but SS2 is about as easy, so that's out. No, I think it's that SS2's environment and enemies are much more unknown and alien to the player. Bioshock's enemies are all people who have gone insane and people wearing huge-ass diving suits. SS2, by contrast, has people who have been corrupted by an alien (well, sort of) race of parasitic worms and alternately warn you to run or tell you that their flesh welcomes you when they see you, psychic monkeys that throw bolts of cold (and later on, heat) at you, women who have been cybernetically modified to act as maternal guardians of the Many's eggs (and have also had their faces torn off, for some reason, not that they needed help with being creepy), protocol droids that ask if they can help you even as they run towards you with the intention of exploding right in your face, eggs that explode when you approach them and produce either alien worms, a swarm of bugs that can't be killed and will follow you around until they die naturally (thankfully they have a lifespan that makes the mayfly look long-lived), or a cloud of poisonous gas, big-ass alien humanoids who are fast and can hit hard, and floating psychic... things that are highly damaging and can only be permanently killed by destroying the brain hidden somewhere nearby. As creepy as Bioshock's crew was, they've got nothing on that cast of horrors.
