Now I know why everybody over the tubes have been raving about it: in short, because it’s brilliant. Absolutely brilliant. Other than interesting game mechanics, which actually didn’t get to me that much, it has a great narrative and one of the best villains ever- GladOS.
In a nutshell, player becomes a young woman caught in an experiment facility, who has to perform different tests using Aperture Science device creating portals. As a test subject, the girl is guided by the omni-present AI (GladOS) with wonderfully distorted female voice and an attitude that could be best described as somewhat deranged. She is both a narrator to some extent, as well as both enemy and companion, it gives player orders, sets rules, tells the story, mocks and teases, promises and deceives – in short, it sets the atmosphere of the game.
There are three pillars to Portal’s success, each spectacularly executed: audio, story and gameplay. The first one is composed of the best AI voice ever together with one of the most wicked ending song up to date (not to mention other excellent sounds). The second is one of the greatest narratives I have seen in a computer game during last couple of years. The third is the game mechanics – mind-bending portal puzzles in hazardous 3D space.
Penny Arcade summed it up perfectly in this comic strip:

No wonder even Yahtzee of Zero Punctuation reviews loves it. The sheer brilliance of this game has resulted in fan-arts, some darn funny, some touching, some kinda risky, some including crossover and some a bit obnoxious.
The best part for me (and I assume many others) was the voice of omnipresent and seemingly omnipotent deity – GladOS. Just like in Manhunt, where main hero is led by a voice of snuff movie maker, who switches between being roles of compassionate guide and cruel torturer, who taunts, teases and comments protagonist’s actions, so does GladOS put Chell into harder and harder tests, gives information (“In dangerous testing environments, the Enrichment Center promises to always provide useful advice. For instance, the floor here will kill you. Try to avoid it.“), which is sometimes warped, mocks her actions later in the game. The writing of GladOS lines is nothing short of brilliant – you can read it on Wikiquote, but without the context it is somewhat similar to reading a movie script.
After trying to kill Chell at some point, this strange relation becomes much more threatening and climaxes during the game’s finale, where two female minds engage in a fight. Delightfully surreal scene is further amplified by GladOS torrent of deranged speech (“killing you and giving you good advice aren’t mutually exclusive“), which she sputters like a being possessed by a demon, altering between smooth sweet voice and spitting threats. You can watch the scene here, but be aware that there are spoilers and the effect is not as powerful without knowing the whole story that has led to that point.

Speaking of the story and ambiance, the narrative is led in a great way, leading some people to overanalyse it. What’s actually interesting is that the story, GladOS and sinister atmosphere of the abandoned experimental facility run by deranged AI was something that resulted from time constraints and actually was not a part of initial design – you can read more about it here.
The icing on this delightful cake (pun intended) is the ending song, “Still Alive”. Unfortunately, it’s not funny at all without first completing the game (duh). There exist many weird renditions of the song and I heartily recommend having a look at them, because some are really hilarious.
In short – if by any chance you have missed this game and you dare call yourself a gamer, you simply must buy it and play it. The sheer amount of songs, jokes, images and cake references this game introduced to the general public is a testament to its greatness. I didn’t expect it when I first started playing Portal, but this is one of these games.
Valve, I take my hat off to you.