The success of the Portal games comes from a combination of two completely different aspects: gameplay and narrative. Portal 2 is, on one hand, a great puzzle game in the classic tradition, sending you through a series of enclosed, self-contained puzzles. These require the arrangement of provided tools to create a single, predetermined "solution" and allow you to advance.

On the other hand, it's the story of the rise and fall of a gigantic research company, and of '60s-style "for its own sake" science -- it even has Flubber! -- told years after the fact, diegetically, by two rogue AIs and a series of tape recordings. And it's genuinely funny!