Does GLaDOS truly despise Chell, or does she truly care about her but unable or unwilling to express it?
Chell is subject to a lot of this as she's a Silent Protagonist. One of the common areas of debate revolves around her silence, as there's discussion about whether she's mute, whether (in the second game) she suffers from brain damage after her experiences in cryo-sleep, or whether she's perfectly capable of talking but just stubbornly refuses to give the insane and annoying robots she's surrounded by the satisfaction of a response. Word of God states the third one is the case in Portal 1, but Portal 2 is ambiguous.
Applicability: Particularly in the first game, as GLaDOS's dialogue provides most of the game's atmosphere and world-building. The creators have encouraged the vast numbers of Epileptic Trees about the series, and about GLaDOS' character in particular. It has provoked a lot of professional analysis and extrapolation about her meaning and symbolism. See for yourself.
Breather Level: After being shot at nearly every time you turn a corner and open a door in chamber 16 you immediately have chamber 17 where the worst thing you may encounter is easily dodgeable energy blasts. And maybe regret over having to "euthanize" the companion cube.
Test Chamber 19 is much shorter than the long and challenging Test Chamber 18, with more forgiving mechanics and less reaction-based gameplay. Subverted, as it ends with GLaDOS attempting to drop you into an incinerator, starting the "escape" levels.
Cargo Ship: Chell/Companion Cube is a popular shipping pair in the fandom.
The Rat Man's graffiti shows he was far too attached to his Companion Cube, and really didn't take having to incinerate it at all well.
The Lab Rat comic proves that he failed to incinerate it, carrying it around with him and even talking to it (including hallicinations of responses) in his schizophrenic mania.
"The cake is a lie" is one of the most famous things to come out of the game, to the point where it's mistaken for The Reveal. Even people who have played the game will quote it due to its Memetic Mutation status, even though the end credits show there really was a cake.
Following the release of Portal 2, many people believe that all surfaces capable of holding portals need to be painted with moon dust. In truth, many other surfaces work too, such as concrete - as seen in the many test chambers for the portal gun built years before the moon landings.
Ellen McLain talks about the experience of being GLaDOS (including a very sexy voice, an unexpected burst of opera singing, and a scary laugh). Have a listen to some commentary.
Cult Classic: Its popularity despite being a pack-in game for a Half-Life compilation owes itself to very good word of mouth from its fans.
Ensemble Dark Horse: The Weighted Companion Cube. It was intended to be this to begin with in the game, but even Valve admitted they vastly underestimated the extent to which it was adopted as a fan favorite.
In a way, the entire game could be considered this. It was initially included as just a stocking stuffer with the rest of the Orange Box as a sort of apology for it taking so long.
Everyone Is Jesus in Purgatory: There have been some truly odd interpretations of this fairly straightforward game as, among other things, a crushing feminist assault on the masculine and misogynist games industry. See the Wild Mass Guessing page for details.
Genius Bonus: Some people have deciphered the morse code tied to the new achievement. It appears to be GLaDOS rebooting.
Good Bad Bugs: While the First Slice demo is supposed to cut the player off after test chamber 10, some players have been able to complete chambers 11 and 12 and get the dual-portal gun. Chamber 13, however, is not in the demo, meaning players are blocked by an eye-destroyingly bright wall of Nodraw when the elevator opens.
Inferred Holocaust: Never mind all the researchers that GLaDOS gassed; "Bring Your Daughter To Work Day" implies that their children became victims too, or were tested to death. GLaDOS also manages to sneak in a reference to the multiple disasters of the Half-Life universe during the Final Battle and in "Still Alive". It's implied during Portal 2 that Chell herself was the daughter of one of the scientists at Aperture and she was present during the massacre.
GLaDOS being a killer AI. The trailers for the sequel don't try to hide this fact.
The fact that you end up escaping the test chambers and making your way through Aperture's backstage machinery.
It's Short, So It Sucks!: Gloriously averted, despite Portal lasting around 4 hours overall and lacking any real replayability. Portal being packaged into The Orange Box - and later sold separately as an Xbox Live Arcade game - was probably a big reason behind the lack of negative criticism for its brief length.
Magnificent Bitch: GLaDOS is computer created by Cave Johnson to run Aperture Science after his death. Once activated, GLaDOS proceeded to kill most of the scientists with neurotoxin and took over the Enrichment Center to conduct tests at her own leisure. In the present, GLaDOS put Chell through a series of potential lethal tests before trying to murder her. Defeated and shutdown by Chell in retaliation, GLaDOS is accidentally awoken years later by Wheatley. Throwing Chell back into more tests, she is eventually overthrown by Wheatley thanks to Chell's actions. With Wheatley threatening to destroy the facility with his own stupidity, GLaDOS convinces Chell to help her take Aperture back from Wheatley, and in turn lets Chell go when that is done. Switching focus toward Cooperative Testing, GLaDOS uses the robots Atlas and P-body to secure control over the lower parts of the facility and eventually give her access to more human test subjects. When a mysterious hacker (actually a mother bird on a keyboard) forces her hand, resulting in the deaths of her human test subjects, GLaDOS sends the robots after it before adopting the baby birds to raise as killers.
More Popular Spin-Off: It's hard to remember that Portal, which technically takes place in the Half-Life universe, started off as a pet project ("a little lagniappe", in McLain's words) that was included as a bonus game as part of the compilation The Orange Box.
Player Punch: "Killing" the companion cube (although incinerating the companion cube is meant more as a parody of this trope.)
Sacred Cow: Like all of Valve's games, no one will let you off easy for disliking it.
Ships That Pass in the Night: Gordon Freeman and Chell is a popular ship due to their ridiculous number of similarities, despite the fact that, as far as we know, the two have never met, and are currently in completely different countries, possibly even in different time periods.
Special Effects Failure: At the very end of the game, when you're lying on the ground outside the entrance to the facility, there is a backdrop of trees laid out in front of you. When robot grabs you for assuming the Party Escort Submission Position, the movement of the screen allows you to see that the tree backdrop doesn't extend past your initial field of vision. This is because the Party Escort robot was patched in long after the game was finished as a teaser to Portal 2.
The Advanced Test Chamber 18 will have you tearing pieces of yourself out in frustration.
Rather notably, Chamber 17 started out as one for the playtesters, as they had trouble adjusting to having to carry the cube around the whole time rather than leaving it behind after one use, and especially couldn't quite wrap their brains around using it as a shield in the opening corridors. The result was the Companion Cube.
The Tetris Effect: Not only the game-fun part, but you'll find yourself wishing you had an Aperture Science Handheld Portal Device for real-life transportation. And planning out the most efficient way to get from here to the fresh-produce aisle. Thinking With Portals, indeed!