Near the end of the game, you find yourself in a wide open room absolutely filled with turrets. Then you start portalling from room to room, yanking them out of their hiding spots and dumping them to the ground. It comes straight out of those movies where the lone hero silently kills off an entire squad.
Surviving the incinerator trap.
Afterwards you find yourself in the bowels of Apeture Science, which reminds you that you've gone against the will of GLaDOS and began on the road to the final confrontation.
Particularly good in this sequence is when you find yourself back in the chamber that GLaDOS claimed was impossible, and now that you have a dual portal gun it's extremely simple. That really makes you feel like you're taking this rat maze Off the Rails.