When Victor Garber takes a break from his usually cool, hard-hitting persona to play Will's dad, he expresses his regret at never going to law school and at the end of the episode decides to apply. This is a Shout-Out to Victor Garber's portrayals of lawyers in many movies and TV shows (Legally Blonde, Eli Stone, etc...)
Victor Garber is also known for his work in Broadway musicals (e.g., Assassins), thus his appearance in a show all about music and musicals is another lampshade in an already overly-meta show.
Rachel auditions for Glee by singing the song "On My Own" from Les Misérables (the musical). Lea Michele, the actress who plays Rachel, made her Broadway debut as Young Cosette in the Broadway cast of the same musical and more recently played Eponine in the Hollywood Bowl concert. For those not in the know, grown Eponine sings "On My Own". Young Cosette/Young Eponine (nonspeaking) rotate nights in most productions, so she would have played both roles.
In the pilot episode, when Sandy sees Will at the linen store, his reaction is identical to that of Ned Ryerson every time he sees Phil in the street. Sandy's last name is Ryerson too (though Word of God says that the name "Sandy Ryerson" was scripted before the actor was cast).
In the second episode, when Terri flicks the kitchen lighter with a very smug attitude (as opposed to her needy, slightly hysterical character) it could be an allusion to Jessalyn Gilsig's role in Heroes as Meredith Gordon, the smug and self-confident woman with the power to control fire and create a small flame from her hand.
Terri's sister asks if her baby will be black, a reference to Jessalyn Gilsig's character (Gina Russo) in Nip/Tuck.
Rachel claims that nudity is one of the few things she won't do to break into the business, which is ironic considering Lea Michele's other famous role involved some nudity on-stage 8 times a week.
In Sectionals, Mercedes sings a song from Dreamgirls; Amber Riley auditioned for Dreamgirls.
In episode 4, Kurt performs Beyoncé's "Single Ladies" in his basement along with Tina and Brittany (Played by Heather Morris). Heather Morris was one of Beyoncé's backup dancers in Single Ladies. Heather Morris actually started out in Glee as only being there to teach them that dance, and then later got the role as Brittany when it turned out they still needed a third cheerleader.
In Never Been Kissed, Blaine, played by Darren Criss, the lead actor and songwriter for A Very Potter Musical, tells Karofsky that he's "Not Alone," the title of a recurring track from the musical.
In "A Very Glee Christmas", Mike wishes for Channing Tatum to stop being in stuff. Tatum starred in the first Step Up. Harry Shum Jr., who plays Mike, had a role in Step Up 2 and Step Up 3D.
In the "When I Get You Alone" number, Blaine tries on some pink sunglasses in a store.
In Silly Love Songs, Blaine tells Kurt about his plan to sing a song to someone on Valentine's Day, leading Kurt to believe that the song is for him, only to later find out it's for someone else. This isn't the first time a character played by Darren Criss has done something like this.
In the episode Sexy, Holly Holliday makes a comment about not liking shows about doctors, in front of Stamos's character.
New Directions does a song from Hairspray in "The Purple Piano Project". Matthew Morrison was the original Link Larkin on Broadway. If you notice, when he hears Rachel's opening notes, he smiles knowingly.
"I Am Unicorn" is full of it:
Burt telling Kurt that, if he can't find any good roles for a kid like him, to "write them yourself". Chris Colfer just wrapped Struck By Lightning, which he also wrote.
The choreography for "I'm the Greatest Star" has Kurt climbing all over scaffolding, hanging from it upside down, and spinning ninjitsu knives. All of which Chris is well known for liking and doing in real life.
Blaine leads the club in a performance of "Last Friday Night", the music video of which he cameos in.
What about Kurt and Rachel's obsessions with Wicked? Don't they notice that April and Shelby look like Kristin Chenoweth and Idina Menzel?
In "Lights Out" Sue sings "Little Girls" from Annie. Jane Lynch with be starring as Miss Hannigan on Broadway this summer.
Colbert Bump: Has had this effect with some of the music they've featured on their show.
Contractual Purity: There is a strange tendency in some people to flip out every time Lea Michele or Dianna Agron appears on a magazine looking sexy. Yes, they are adult women in their mid-20s, not teenagers, yet somehow the fact that they play minors on TV means that they are "sending girls the wrong message" every time they so much as wear low-cut shirts. Glee is an unusual case, though, since it is not a kids' show and it engages in sexual and other adult themes on the show, so this expectation is solely due to viewer misconceptions.
Dawson Casting: All of the kids' actors are well in their 20's, with Cory Monteith (Finn), Mark Salling (puck), and Harry Shum, Jr. (Mike) being the oldest and only a couple years younger than their teacher. The closest exceptions are Chris Colfer (Kurt), Charice (Sunshine), and Damian McGinty (Rory), who were all in their late teens when their characters debuted.
A lot of fans were disappointed that that the assigned Puck/Mercedes duet in Episode 10 never happened. So in Episode 18, they not only let them sing together but had them date as well.
Shelby Corcoran as Rachel's biological mother. Nicely they kept us off the trail by making her the rival coach and head faking toward another Love Interest.
Subverted in that a lot of people wanted an Odd Friendship to develop between Quinn and Rachel, especially for the homeless Quinn to move in with her. Such friendship and moving in did occur with Quinn-just with Mercedes.
In Bad Reputation, Rachel told Puck that most of the school referred to them as "Puckleberry" while they were dating.
Pretty much the entire opening sequence of the second season premiere is a nod to the biggest complaints of the show. And it's awesome. The only problem with that? The show kept on doing that even when the complaints started being legitimate.
Various characters in Duet calling Kurt out on his previous behavior in Season 1. It's like the writers read the Headscratchers page.
Furt gives us Finn mentioning "Puckleberry," "Finchel," and "Furt."
"Silly Love Songs" gives a nod to a different fandom during the Warblers' rendition of "When I Get You Alone". At one point, Blaine and two other singers grab sunglasses from the Gap's sunglass rack; the glasses that Blaine wears are Darren Criss's iconic pink sunglasses.
"Rumours" has the club rattle off all of the fandom names for the official couples - and create two new ones. Pizes (Puck/Lauren) and Tina Cohen-Chang Chang (Tina/Mike). The actors themselves are in on it as Mark Salling gave Puck/Artie a name (Partie).
"The Spanish Teacher" acknowledges a common fan complaint that on the rare occasion we hear Will speak Spanish, his accent is appalling considering he's teaching it in a high school.
If you pay attention during "Swan Song", you can see Ellis Wylie of The Glee Project - eliminated in her second week and back as a background extra that even Ryan wasn't aware of.
So You Think You Can Dance finalists have a habit of appearing on the show, mainly in competing show choirs (where the singing is actually done by a separate group of singers), although Courtney Galiano has been credited for having a speaking (non-dancing) part. At least one SYTYCD finalist has appeared on Glee before SYTYCD.
Hey, remember that time Finn Hudson got eaten by a Wendigo?
Kurt's dad is Mike O'Malley (meaning, he used to host a Nickelodeon game show that was kinda like a kiddie version of American Gladiators — and later, had a spinoff featuring kids from other countries).
After Idina Menzel was cast, Ryan Murphy insisted she would not turn out to be Rachel's mother. Pretty much no one bought it.
Ryan Murphy also said that Finn and Rachel would stay together in Season 2. "Special Education" pretty much did away with that.
When he accepted a GLAAD award on behalf of the show, RM promised the gay audience that Kurt would never be the victim. Cut to season 2, in which anti-gay bullying becomes a huge plotline. Of course, that was before the rash of well-publicized suicides due to bullying. Even those gay advocates who completely supported RM's initial standpoint agree that the way schools deal with homophobic bullying needed to be addressed, and that Glee did it perfectly.
YMMV on that "perfect" part.
The entire "who is going to be Kurt's boyfriend" debate is a perfect example of this trope. From implying it'll be Sam to saying "Chord is definitely not the boyfriend" to "BLAINEBLAINEBLAINELOOKATBLAINEVERYONE" to "We haven't decided yet" (with the latter two included in the same interview, no less), well, somewhere in there there's several lies, though it remains to be seen which statements are lies and which are truth.
As of Original Song, Kurt and Blaine are together.
Although as of The Breakup, they are not.
They're back together as of "Love, Love, Love" and engaged. This pleased the Klaine fanbase, and only them.
Rule 34: There is a HUGE section of fandom that revolves around not only Les Yay but one of the girls (usually either Quinn or Rachel, but sometimes Santanna, or occasionally Brittany) secretly having a cock. Bonus points goes to the "were-peen" subgenre where it only comes once a month in place of their period. No points for guessing how Quinn's pregnancy turns out in this universe.
What Do You Mean, It's Not for Kids?: The way so many "family values" groups reacted to the "Push It" number and the negative portrayal of the celibacy club in Episode 2 is a bit more understandable when you realize that a lot of parents of younger children were under the mistaken impression that Glee was High School Musical: The TV Series. (Which had nothing to do with the marketing of the show, but everything to do with the surface similarities - musical comedies set in high schools! - and general public ignorance about musical theatre not always being a "family friendly" medium.)
If the Parents Television Council's uproar about the GQ cover and the more recent one lambasting Lea Michele's choice to appear in a low-cut photo on Cosmo are anything to go by, people are still not getting it. This was even unintentionally lampshaded by the people in the latter case, who said "Lea Michele may be an adult, but to pretend that she doesn't know her fans are 11 is just ignorance." Why should an actress on a show that has had subplots about teen pregnancy and when to lose your virginity, jokes about oral sex, a former teacher turning to drug dealing, and students taking uppers to do better in competitions be expected to cater to 11-year-olds?
It doesn't help that Claire's, a store famous for cheap jewlery aimed at pre-teens, now sells a small ammount of Glee merchandise. Including make up sets for Rachel and Quinn, Team Finn and Team Puck necklaces, bracelets, and even knee socks.
Glee was at first conceived as a movie by Ian Brennan, he wrote the script in 2005 but it never got anywhere, in 2008 the script fell into Ryan Murphy's hands and he and Brad Falchuk (who worked with Ryan on Nip/Tuck) decided the concept would be better as a tv series
Originally the creators where going to include a character named Rajesh. However, Chris Colfer made such an impression at his audition (for Artie) that the character of Kurt was written specifically for him.
The producers originally wanted a black girl in the Britanny role (who was then only ever intended to be a background character) but Heather Morris was given the part after teaching Chris and Jenna the Single Ladies dance.
The Don't You Want Me duet between Blaine and Rachel shows what a lot of duets would have been like if Darren Criss was cast as Finn, the part he originally auditioned for.
Aaron Tveit also auditioned for the part of Finn. Some of the more Broadway-obsessed portions of the fandom wish he had gotten the part.
Sam was originally intended to be the Kurt's romantic interest, but at the last minute the writers decided Chord's chemistry with Dianna Agron meant he would be better as Quinn's boyfriend. At the end of season 2 he and Mercedes had a Relationship Reveal that was supposed to continue on into the next season, but unfortunately Chord declined to come back after his option as a series regular wasn't picked up.
Then another WCHB instance cropped up after the writers introduced a highly likeable side character as Mercedes' love interest, shortly after which Chord did return and whatever plot line involving Mercedes and Shane were sidelined to bring Samcedes back into being.
Telly Leung (Wes' actor) originally auditioned for Blaine.
Naya Rivera (Santana) had originally auditioned for the part of Mercedes.
Through no one knows what would have happened had Cory lived, Ryan Murphy had a plan. While not giving away any specific plot points, spoilered just in case:
Murphy: The final year of the show, which will be next year, was designed around Rachel and Cory/Finn’s story. I always knew that, I always knew how it would end. I knew what the last shot was, he was in it. I knew what the last line was—she said it to him.