Renders the first game obsolete. That means the game is flawless.
There are very few examples of perfect games, because making a perfect game is hard.
System Shock 2 is an example of a perfect game. The guys at Irrational, despite so many limitations, made a flawless game.
SS2 is the sequel to genre bender System Shock, in case you couldn't figure that out...and in case you didn't. Please pay better attention in class, because your dumb.
The genre bending in System Shock 1 was very light, yet still there. And it was one of the first story driven FPS games (cough, Marathon.)
System Shock 2 is an RPG/FPS/Survival Horror game. As diffucult as that may be to get through your head, and it will be, you'll soon find yourself completely used to the games mechanics. The RPG elements are very strong and effect the way the game is played.
Psi powers make the game interesting, as I found myself activating a power that hurts anything in a 5 foot radius all the time.
So in short, if you can find system shock 2, get it. It IS worth your time.