Did she really delete part or all of Caroline? If she did, was it just a matter of deleting Caroline's memories, or did she murder an actual consciousness? Or was it not a murder but a Mercy Kill?
Does she see Chell as her lover, daughter, friend, enemy, or some combination of these?
How much of her crazy do we attribute to the mainframe, the personality cores, the trauma of Caroline's uploading, and/or Caroline's original personality (after all, she didn't seem to mind Cave's Comedic Sociopathy...)?
How separate are GLaDOS and Caroline, anyway? When she speaks of Caroline as a separate person, is she just trying to distance herself from emotions she'd rather not have, or is there more to it? Is it even a meaningful question now that we know her personality cores are fully sentient on their own? If they're truly separate, is that something that developed over time as GLaDOS lost her memories and had personality cores added on, or were they always two people? Did GLaDOS have a fully sentient mind of her own even before Caroline was shoved into it? If Caroline's always been a separate person, was she dormant all this time, or conscious but unable to communicate?
Is PotatOS really unable to lie? If it was true when she said it, is she still unable to lie after the extra jolt from the portal gun?
When, at the end, GLaDOS says "And killing you? Is hard.", is that because she's grown emotionally attached to Chell, or is just admitting that Chell is impossible to kill?
Did the turrets fake-out their Bolivian Army Ending and instead sing Chell a farewell song on GLaDOS's orders, or in spite of them?
During the ending song, when she sings "Goodbye my only friend. / Oh did you think I meant you? / That would be funny if it weren't so sad", is she still mocking Chell, or is this a tacit admission that Chell really is her only friend?
As with GLaDOS, how much of his turn to insanity can we attribute to the mainframe's influence? It cannot have had more of an influence on him, which means the starting points may have been different. He expresses huge delight in his newfound power and comments on how tiny and insignificant Chell is now. His traps are far more deadly and while GLaDOS is only testing Chell with timed experiments as revenge for killing her, Wheatley wants to kill her to keep in power. Even if being plugged in rattled his circuits more than he'd expected, he does hint repeatedly that he dislikes humans and seems to treat Chell as a means to an end regardless.
The exact nature of his stupidity. Is his ability to come up with decently clever plans because they will result in the dumbest outcome, the facility exploding? Or is he just another Aperture Science invention that doesn't work the way it is supposed?
Was being plugged in Wheatley's plan all along? It's not like he would have grown legs and been able to walk if he'd gotten out - Chell would still have had to carry him everywhere.
What if Cave Johnson is actually a freaking genius and natural/scientific philosopher, and the rest of the world (especially Black Mesa, who practically ended the world) is instead floundering in the gaping maw of madness?
Awesome Ego: "Rick" the corrupted Adventure Sphere is so self-absorbed in his own ego that you can't help but love him for it.
Big-Lipped Alligator Moment: The turret orchestra that plays during Chell's ascent to the surface at the end of Portal 2. There is some minor foreshadowing, but it's out of the way. In Chamber 16, prior to Wheatley helping you escape, there's a vent guarded by a turret which leads to four turrets practicing. You can come up with some justifications - the lyrics seem to suggest it's GLaDOS and/or the turrets and/or Aperture as a whole bidding a fond farewell to Chell, who grew up there - but that doesn't make it much less bizarre.
There are two real camps of Portal 2 fans, those who find it to be an excellent game and often say it's one of the best games ever, and those who think it's far too short and the focus on Co-op and relative ease of puzzles makes it simplistic. The former are decried as newbies to the Portal franchise who don't understand what made the first game great and the latter are decried as "Stop Having Fun" Guys who can't enjoy the story.
Hasn't exactly sparked too big of a flamewar over it (yet), but there is definitely a Broken Base regarding Wheatley's voice. Was Valve wrong to replace Richard Lord, or did they make a good choice hiring Stephen Merchant? It is worth noting that Richard Lord was never supposed to be Wheatley's voice actor. Richard Lord is a programmer for Valve, and did the voice as a stand-in for the E3 demonstration. When Wheatley turned out to be so popular during the demo, Valve went out to get big-name Stephen Merchant.
Chell/Wheatley is growing increasingly popular. Most of the fics based off of it either involve Wheatley sometime After the End, desperate to apologize to Chell for what he did and suddenly finding a means to do so, or Human!Wheatley, typically based off of Stephen Merchant.
The GLaDOS/Chell ship (which was already popular after the first game) is still going strong here.
The cores are often shipped with each other. Space Core/Curiosity Core ("Spaciosity") or its rival Fact Core/Curiosity Core ("Factiosity") have become quite popular, as has Fact Core/Adventure Core ("Factventure").
GLaDOS is also often shipped with The Sniper of TF2 fame. The gag for it being that their voice actors are married in real life.
Crazy Awesome: Cave Johnson. He is insane, but this leads to him making some truly great things. Don't want to deal with your workers leaving their cubicles while on duty? Corral them with Frickin' Laser Beams. Think the advance of technology is too slow? Take it leaps beyond your current decade just because you can. Don't like dealing with the laws of physics? Invent your own! And of course the one concept that started it all: don't like dealing with mildew-growing shower curtains? Create a quantum tunneling device that lets you instantly travel between any two surfaces you want.
Discredited Meme: Valve said that they don't include cake in Portal 2 since they got tired of The Cake Is a Lie jokes. There's still one reference to it though: GLaDOS tricks you with a door marked as her emergency shutdown and cake dispensary. One of Doug Rattmann's murals at the beginning of the game also shows GLaDOS holding cake out to you.
Draco in Leather Pants: GLaDOS, to some extent. While there are still a large contingent of players who love her for being crazy and evil, there are a few who downplay or straight-up ignore all of her actions from before being put in a potato to characterize her as a loving mother figure, instead of the verbally abusive and homicidal AI that she has been.
Ensemble Dark Horse: The Space Personality Core. Imageboards and Youtube comments in particular around the time the game came out were inundated with screams of SPAAAAAAAAAAAAAAAAACE.
Esoteric Happy Ending: The co-op ending is very silly and upbeat, having GLaDOS finding a horde of human test subjects. Very good for GLaDOS. Not so much for the test subjects.
Even Better Sequel: In the first game, the most emotional sequence besides the boss battle is dropping a box down a hole. In Portal 2? We've got so many characters introduced and developed, that it's practically a different world entirely.
Fanon: After the release of Portal 2, it was revealed that GLaDOS was once human, or at least shares a body with Caroline's consciousness, which naturally made many people believe that all cores were created the same way. It's also believed that Wheatley's human form greatly resembled Stephen Merchant.
Virtually everybody assumes that Chell is the daughter of Aperture's founder Cave Johnson and his secretary Caroline. There is evidence to suggest she was the daughter of someone at Aperture (her apparent signature appears on a children's science project in the 'Bring Your Daughter to Work Day' display) and the assumption is sort of understandable since Cave and Caroline are the only other human characters named in-game and GLaDOS, who has Caroline's mind inside her, is obsessed with Chell to an almost mother-like degree, but nowhere does it imply she was anything near as important as the daughter of the founder. Plus, Cave Johnson died in the early 1980s and was already an old man◊ at that point (he looked like this circa 1943). In the first game, Chell looks like she could maybe be a young-looking forty-something at oldest, and in the higher-quality graphics of the second she doesn't look a day over 30. Cave could have invented something to keep his daughter young or himself and Caroline fertile, and he's crazy enough to force people to spend years in stasis or something, but you still have to be really trying to interpret him as Chell's father to make it work.
There is also a common belief that Chell has no memory of anything before Portal based purely on the fact that we are given no information on her backstory, and possibly the fact that she seemed to not know GLaDOS was evil. The idea that she cannot speak, or has difficulty speaking, is also common despite Word of God specifically saying otherwise, although this could be seen as a justified belief, as Valve has shown that she avoids every opportunity to speak, even in the Lab Rat comic.
At the time of the game's release almost every word out of Cave Johnson's mouth became a meme of some kind.
The Space Core is just as quotable as Cave Johnson.
Game-Breaker: There's a minor example for those with the Razer Hydra motion controller. The extra features added for Sixense's map pack work in the main campaign, though not in co-op. While the tests are designed in such a way that you usually can't reach the buttons unless you can walk to them in the first place, and the gel sections generally don't need precision, the ability to place cubes at a distance does make your job somewhat easier. It also makes placing the cores on Wheatley a breeze, since your don't have to jump on the Repulsion Gel. Conversely, Thermal Discouragement Beams can be a lot harder to aim.
With the addition of the Challenge maps in the DLC, the Hydra has become a true gamebreaker. With the ability to move portals and boxes, it's possible to beat courses with less portals than should be possible (without outright cheating).
The "lemons" rant is funnier when you realize citrus fruit oils are, in fact, highly flammable.
Cave's comment in one of the trailers about how turrets fire the whole bullet when shooting is even funnier if you know a bit about how guns work and pay attention to the mechanisms seen. The turrets appear to use a spring-loaded piston, which is impractical in a gun. At worst, a single bullet would give the target a bruise, which rather well explains how Chell can take over twenty bullets to the chest before going down.
Good Bad Bug: A design oversight in the first half of co-op chamber 5-5 allows you remove the cube from there and take it with you to the second half, completely bypassing that portion of the test.
One of the taunts Wheatley gives for making you jump down the pit is the supposed pony farm he has down there. If only Valve knew of a certain Periphery Demographic of a certain cartoon show that suddenly was popular about the time of the game's release... Then this◊ came up to mock this very concept. And let's not forget that at the end of the game, Wheatley gets banished to the moon. It turns out Gabe Newell is a fan of the show!
The Space!Cave universe in the Perpetual Training Initiative, where said Cave keeps getting frustrated at his subjects attempting to "uncover the conspiracy" of being in space (and often getting killed in the process) despite him mentioning they're in space every half hour. What was likely originally a reference to the big twist in Dark City has taken on a new dimension with the release of a game with a similar twist less than a year later: Inversion
After playing the absurdly irresponsible portal scientist Cave Johnson, JK Simmons went on to play the Author in Gravity Falls, another scientist who works with a portal, and is extremely serious about the safety issues involved.
This piece of fanart (or, more specifically, the first two sentences in the artist's comments) predicted part of GLaDOS's backstory in 2009.
One thing Computer!Cave Johnson does to try and stave off boredom is to add ghost-busting to every work of fiction on Earth. Come 2015, we get to see the Ghostbusters themselves (and many others) visit Aperture Science and even work alongside GLaDOS in LEGO Dimensions.
One of the alternative Caves' claims about not having to prepare for Godzilla because Godzilla only attacks Tokyo became doubly hilarious after the King of the Monsters devastated San Francisco and parts of Hawaii in Godzilla (2014), with additional cities on US soil falling victim to the MUTOs in the same film.
In addition to the countless Aperture Science personnel who were killed, Wheatley also mentions that besides Chell, there are at least 10,000 other test subjects being kept in stasis, and they're most likely all dead.
At the end of the single-player campaign, Chell is abandoned in a field of grain with nothing but a Companion Cube for company. Depending on how the world of Portal interacts with the Half-Life universe, she could be very screwed. The humans presumably beat the Combine within that time, and they seem to have accomplished some rebuilding since the wheat field appears to have been actively cultivated, but who knows what else is going on? However, it's also inverted: since Earth's atmosphere is still breathable and there are still birds and plants, we can infer that the Combine was defeated before they managed to xenoform the planet.
Given Cave Johnson's progression from recruiting the "Best of the Best of the Best" as test subjects in the 50s to street bums in the late 60s and finally his own employees (and their children, no less) in the 80s, it's very strongly implied that most if not all of them died.
Wheatley tells you that the turrets are programmed to feel pain, of a sort. How good do you feel about all those turrets you knocked into pits, incinerated or carried through Emancipation fields now?
Chell's been continually exposed to the various toxic substances that Aperture Science created, especially during her sojourn in the old test chambers. How long is she going to last, really, after the game is over?
At the end of the co-op campaign GLaDOS gets her hands on a huge supply of fresh human test subjects. After making it abundantly clear that as far as she's concerned, it isn't proper science if the test subjects don't die horribly.
In the "Art Therapy" DLC, she says she's been trying to turn them into "killing machines", only to find that they were "surprisingly fragile, and surprisingly vocal about how fragile they are" compared to "you-know-who". Now they're all dead. Bonus points for the fact that occurred in the span of one week.
The blue screen for the reactor prompts the user to press any key to vent radioactive material into the atmosphere, GLaDOS likely did this because the reactor was in the final stages of meltdown at the time and wouldn't go out of the way to save human life.
It's Short, So It Sucks!: Portal 2 was said in prerelease materials to have a 10 hour single player and 10 hour co-op mode. In the game proper, each mode can be beat in somewhat less than that.
On a smaller scale, the final boss battle has been critiqued for the time limit being too short to allow you to hear all the dialogue in one playthrough.
Cave Johnson. We get to listen to him descend into bitterness and despair shortly before his death.
Wheatley, that lovable idiot, floating in space potentially forever. He says he's sorry, and how much of it was ever really his fault?
GLaDOS. Apparently spent the time between games reliving her own destruction over and over and over again, if you believe her own report. Has her head painfully ripped off her body while she screams in horror. Attached to a potato battery purely as humiliation. Has to listen to someone she knows to be a complete moron gloat about his victory over her and watch him ruin her beloved facility. Oh, and we find out how maddening the influence of the personality cores and the mainframe must have been all this time. And that's without even getting into the whole Caroline backstory, which is pretty sad and traumatic no matter how you interpret the details.
Misaimed Marketing: The advertising, tie ins, and just about everything heavily tout Co-op, yet what is this huge page about? Not to mention according to this, only about half of PC Portal 2 players even start co-op, and only a fourth finish.
Narm: GLaDOS's scream of agony while she's being forcibly ripped from the control center by machines is supposed to be scary and shocking, and for many it is... but the thing about her voice is that when she screams in agony, it sounds like she's an overly-autotuned singer.
Newer Than They Think: The "Cara Mia" Turret Opera, though you might think it was some old aria the developers borrowed for the sake of a big finish, was actually written for the game by composer Mike Morasky. Ellen McLain (a trained opera singer) used her knowledge of Italian to come up with the lyrics herself.
Robo Ship: The two cooperative mode robots were being shipped together by fan-artists before we ever saw videos of them. Images like this◊ helped.
"Stop Having Fun" Guys: Admittedly, it's fun to mess around with cheats every once in a while, but then you have a staggering amount of Jerkasses who will only play co-op with cheats enabled and deride anyone who doesn't want to use them.
That One Achievement: "Professor Portal", which requires you to play through the co-op Calibration Course with a friend (as in an invited one) who has never played before, is the bane of everyone looking for 100% completion. There are threads on every dedicated forum searching for people who qualify so others can unlock the achievement. It has the third lowest completion percentage of all achievements, trailing behind "Friends List With Benefits" (hug three friends) and "Still Alive" (complete Course 4 without either player dying).
Speaking of which, "Still Alive" is rather difficult to pull off, even with two players that communicate well. Most of the chambers punish screw-ups with death, forcing you to go back to the first chamber and start again. The final chamber is all that and more, since it's filled with turrets, spikes, and pits.
"Schrodinger's Catch" in single-player mode is entirely a Luck-Based Mission. To get this achievement, you have to catch the caged box covered in Repulsion Gel in Chapter 6 before it hits the ground. However, the box's trajectory the moment it breaks out of the cage is completely random, so there's no way of knowing where the box will go. You have to be really lucky and hope that it comes your way to get this achievement.
The final puzzle of the eighth chapter tends to be the only one that completely stumps everyone to the point of having to use a guide. You have some turrets that you can't easily get behind or past, there are no cubes to take them out, but there is repulsion gel. Up to this point, you usually just had to make a portal to redirect where the gel travels and splash it on the turrets. Here, though, you have to get creative with an excursion funnel...which would be a lot more obvious if its reverse switch wasn't way far and away from the button you have to press to drip out the gel. This puzzle also hands down requires the most timed steps out of any puzzle in the game, meaning it's not one that easily runs step-by-step; it's a long sequence of steps that must be done in order one after the other. And it also doesn't help this chamber also has an achievement inside it that requires you to violate some of the established game rules to complete it!
Additionally, some of the in-between sections during the second act can be a real pain, since they require extremely long distances of traveling and tediousness, and are often games of finding that one tiny white dot you can stick a portal onto.
They Wasted a Perfectly Good Character: While Caroline being GLaDOS' original persona is a good concept, it falls flat when you realize that Caroline's presence only amounts to five lines which really don't give much of an impression. It could have been interesting if we got a deeper exploration of her character and see where similarities and differences between GLaDOS and her are.
It's hard not to feel bad for those Frankenturrets. Especially when they look at you with big eyes, shivering after you picked them up.
The defective turrets. They're blind, bulletless, casing-less, and fully aware they're defective. You also have to feel kind of sorry for the functional turrets that get rejected, shouting, "I did everything you asssskeddd" as they get incinerated.
The "Space" Core. All it wants is to go to space. And it gets its wish!