Hands down, the best episode Glee has ever done.
For a show as inconsistent as Glee has been, this episode nailed nearly everything perfectly. Let's start with the title. "Duets" suggests people coming together but is, of course, an ironic title. The episode's overriding theme was loneliness. Because loneliness deals with individuals, let's go with each character's story in this episode.
Artie: Never my favorite character on the show, I love how Artie was handled in this episode. Artie's social awkwardness is usually shown as having a direct connection to his physical handicap - because he's confined to a wheelchair other people avoid him and thus he never gets an opportunity to develop the same social skills others have. In "Wheels" it was pointed out, almost as an aside, that Artie travels everywhere with his parents, though the consequences of that on his, and thus the team's, socialization were a prominent point of the episode. From the first time we saw him, and that awful child's haircut, we've known that his parents control almost every aspect of his life, but in "Wheels" we saw that it was at the expense of his peer interactions. Artie's relationship with Tina seemed less an attempt to break free from his home socialization and more an attempt to develop "normal" teen interactions. In "Duets" we see Artie trying to replace Tina with Brittany but he realizes that even being close to someone physically doesn't mean you develop genuine closeness. As sad as this realization is, it's commendable that he refuses to lower his standards just to have something.
Quinn: Another character who I didn't particularly care for initially (nor were we supposed to), I've come to love her long-term character arc. In "Home" she took time to care for Mercedes, exposing her own fears and vulnerabilities in order to persuade Mercedes to be healthy. At the time I thought it was just a throw-away segue to justify alignment shift, but in retrospect something else was going on. Something much deeper, and something I didn't realize until "Duets." Ever since "Home," Quinn has been depicted as becoming wiser, more conscientious of people around her, more caring, and even nurturing. In "Duets," though they never directly speak about Quinn's pregnancy, it is a secondary point of her interactions with Sam, and that's when it hit me: Quinn is a mother now. More specifically, she's a Mother, in the sense of, "... Maiden and Crone." This is a much more interesting role for her than Blonde, Bitch Girlfriend, and I'm looking forward to seeing how it develops. Her burgeoning relationship with Sam, and her realization that she's not the Crone and that she shouldn't cut herself off from people, that she needs to engage in the duet structure everyone around her is wise enough to know they need even if they're too foolish to pursue it responsibly, is wonderfully done. For the first time, I want to see more of Quinn.
Kurt: Oh man, what a great Kurt episode. Glee's depictions of teen homosexuality have been handled respectfully without putting the poor, prejudiced gay kid on a pedestal. It's a very delicate balancing act and the show is fantastic at showing that Kurt doesn't deserve biggotry because of his sexuality, but at the same time he's not a very nice person. "Duets" was the first episode to acknowledge how much he screwed up things with Finn and that while Finn was out of line in his reaction, Kurt was out of line in his provocation. At the same time, this episode not only acknowledged that Kurt has a right to the same opportunities for partnership that everyone else should have, it how much harder it is for him to pursue those opportunities than for his straight counterparts. Your heart breaks for Kurt in this episode, and is both sewn together and ripped apart again in the scenes with his father, Burt (though to be fair, Burt is such a wonderful man that any scene with him makes you want to cry in joy and sadness, sympathy and euphoria, whenever he's interacting with his son). I love the final part where he sings with Rachel as he accepts that even if he has to be lonely, he doesn't have to be alone.
Brittany: I admit, Brittany was the real reason I wanted to write this. While we enjoyed her in the Britney Spears episode, "Duets" was the first time she actually became a full character. Having her make out with Santana made for a fantastic counterpoint to Kurt's homosexual issues, all the more so because it was unstated that girls experimenting with lesbian or bisexual urges were okay with our society because it's sexually arousing to heterosexual men, but male homosexuality is not okay because homosexual men find it threatening. I hope they don't develop her into a lesbian but it would make for an unexpected, and not altogether negative, twist (is that what she meant in "Audition" when she talked about touching Coach Beiste's breasts?). I don't think the point of the scene was sex, however. In this episode, Brittany is constantly shown as lonely. Santana, her closest friend, admits to using her for nothing more than a digestive aid (and, implied, a sex toy). While her approaching Artie was unexpected, in many ways it makes sense. Brittany wants a Nice Guy, and on the surface Artie fits the bill. She wants someone who wants her as much as she wants them. Remember, Brittany is a character who can't lie - the only time she tries is in "Audition" and she breaks down in ten minutes before revealing too much information. Brittany really did want to be with Artie but not for the reasons anyone thought and she didn't know how to tell him. I choked up watching her nose the meatball across the table at an empty space. Oh Brittany. Oh my heart!
Sam: I almost wasn't going to talk about Sam. I love how he's being set up as a hero. Sam is honorable in "Duets." He wants to keep his word to Kurt. He refuses to engage in high school politics even after he sees the consequences of his actions. He admits that he's in Glee Club for the popularity (poor, misguided fool) but he doesn't let pursuit of that popularity drive him to distraction. I love Sam in this episode because he's as lonely as everyone else but he goes about the right way of fixing it.
Finally, "Duets" had some fantastic musical numbers. While "The Power of Madonna" had the best music and production numbers of any episode so far, "Duet" is a very close second and didn't get that position by relying on nostalgia or gimmicks. Most of the songs were ones I was completely unfamiliar with but were executed perfectly. "Sing!" was my favorite, but "River Deep, Mountain High" may be the most powerful, most inspired, best sung, and best danced number they've ever done.
The only place this episode fell short was Sam and Quinn singing "Lucky." I understand that the director was trying to show that a sincere, understated performance was more touching than all the brouhaha everyone else was doing, but it didn't ring true for me. I'll still take "River Deep, Mountain High."
Even if you haven't watched the show to this point, consider picking up with this one and trying this episode out.