After Wheatley takes over GLaDOS' body, he rattles off a line in Spanish, claiming even he does not understand it. In English it translates to, "You are using this translator software incorrectly. Please, consult the manual."
"It ain't over till the fat lady sings" is a well-known phrase, based on the stereotype of opera performances casting larger women singers in major roles. Staying true to the old saying, Portal 2 concludes with an opera song, sung by a uniquely fat turret with a feminine voice.
Initially the game was going to be a prequel to Portal rather than a sequel. The character Chell would be replaced with a character named Mell and Cave Johnson would appear rather than just be heard in a series of audio recordings and a new style of game play would have been invented with no mention of the portals. Negative feedback from play testers changed development focus from a prequel to making the game a direct sequel.
GLaDOS may be named for real life Dr. Gladys J. Loring who was one of the psychiatrists who evaluated Mercury Astronaut candidates at Wright-Patterson. These men were put through extreme tests that pushed the boundaries of ethical testing.
In the lead up to the game's release, numerous edits were made to the original Portal, to both accommodate Portal 2, and to embed teasers for it, right inside the original. These edits were automatically patched into all copies of the game by the Steam software. The most notable change to Portal was that the ending was retconned so that now Chell *doesn't* escape the facility, but is instead dragged back inside by a robot, right after defeating GLaDOS (Hence explaining her need to escape all over again in this game). Another significant change was that radios were added to all the original test chambers. Each one of these radios, if placed in a certain location, will emit SSTV sounds, a primitive form of television broadcasting, which if decoded, provide users with various teaser images associated with Portal 2. Incidentally, a similar SSTV easter egg is hidden somewhere within Portal 2.
Cave Johnson's face is based on the appearance of Portal 2's Lead Animator, Bill Fletcher, and his assistant Caroline's was inspired by that of Valve Visual Designer, Laura Dubuk. In a hidden room you can see a portrait of the two of them created by Jeremy Bennett, another artist at Valve (Caroline's was artistically aged because Dubuk is a good deal younger than her in-game counterpart).
In one of the commercials for the game (narrated by J. K. Simmons playing Cave johnson) you can briefly see an exploded diagram for the Portal Gun. One of the central components is a miniaturized Black Hole.
The PC version of Portal 2 has a classical secret. If you are playing fan made levels released to the Steam Workshop, most of the time, the background music that plays is an electric remix of Prelude in C Major No. 1 by Johann Sebastian Bach.
Additional information in the form of trivia may be acquired via Steam in the form of a digital book titled "Portal 2 - The Final Hours" via Steam's Store for $1.99 USD. To respect copyright laws, none of the information will be repeated here.
The trivia item below may give away important plot points.
It's not exactly clear how much time Chell has spent in stasis between events of Portal and Portal 2. At the beginning of Portal 2 she's woken up first time after 50 days of sleep, for mandatory set of physical and mental exercises. Next time she wakes up, the announcer fades out (or stutters) after uttering 5 nines, which could either mean that Chell was sleeping for at least 273 years or that the announcement system reached a maximum. It may alternatively have broken after much of the facility fell into disrepair after the defeat of GLaDOS at the end of Portal which could also mean that she might have been sleeping for less time than announced. However, reading it in a different way, since the computer was stuttering over only the number nine, but had in fact been able to properly convey the number fifty, it could simply be that Chell was in stasis for any number beginning with the actual word 'nine', thus giving her a possible range of stasis anywhere from three months to many, many years. Later, in "The Final Hours of Portal 2", the time span is revealed to be 50,000 years.