The first appearance of the MechaSkaarj, which seems to be a Shout-Out to the third level of Unreal: the lift you are on judders to a stop, and again all the lights go out. Something lands heavily on the lift roof, and begins to cut the cables. Again the alarms blare, and as the lift drops out of control you see the Skaarj reflected in the sparks coming from his Wolverine Claws as he digs them into the wall to slow his descent.
The suspenseful scene just before the appearance of the Izarians: a body is dragged under a door as you run to help, then one of the Izarians darts Alien-style across a corridor, complete with Scare Chord.
The way the Araknids batter their way through the air vents on Hell is quite cool as well.