Portal, while a fantastic game, is guilty of committing one of video games' greater taboos: it left players wanting more. Not due to how mind-blowing the experience was, but because it was too short. Sure, you could play through it again and again. But after the 80th time, redundancy sets in.
Chasing yourself in the corner loses its fascination. Euthanizing your Weighted Companion Cube may grow to become second nature. And dealing with GLaDOS may become no more irritating than working with Vista. All that once captivated, seemingly loses momentum.