Being that the game was developed in 1999, the models of the enemies aren't complex, but it's one of those cases where the technology actually works in its benefit, since it abstracts their horrific designs to look even more inhuman. (Fan made modification recreate their models with more polygons, but the increased detail doesn't work in their favor.)
Moreover, the sound design is fantastic. You can hear the ramblings of the mutants or the terrifying screeches of the cyborg midwives even if they're just lumbering around, hidden around the corner or cloaked in the darkness. Rarely has the chattering of monkeys been so terrifying; you know there's something in the room waiting to attack, but where?
The music is different from the original, with most of the soundtrack consisting of ambient computer noises and low-key synth, but it kicks into higher gear with techno and drum and bass during combat. It inflicts anxiety even when just wandering around, even if it's occasionally jarring.
Despite feeling like a drastically different game, System Shock 2 is still a clear evolution of the concepts of the first game. While that game definitely aged, a product of its era, the sequel is just as playable today as it was in 1999. It's dark, scary, compelling, and due to all of the possible character rebuilds, extremely replayable.