
"Date of conception" is what D.O.C. officially stands for, "Desmond Off'ed Charlie" is a possibility I made up on my own. Sounds crazy? Well, if Desmond hadn't been such a Mikhail sympathizer in this episode Charlie would've made it out of the Looking Glass alive!
Where was I? Ah, yes, D.O.C., the episode which makes us wonder how many simultaneous secrets Sun can keep from Jin. The good news is her motives are slightly more pure this time around, at least in the flashbacks.
Jin's mother, a prostitute, meets Sun and demands $100k to stay quiet about Jin's "shameful" past. I guess this is where cultural differences come into play, for if I were in Sun's shoes I probably would've rattled off a few expletives and pushed her into the river. Instead, Sun meets with Jin's father (Secret #1) and then scores the $100k off her own father (Secret #2), which is the underlying reason Mr. Paik promoted Jin to his personal thug (Secret #3) as part of the arrangement.
On the island, the news of Juliet's role with the Others starts spreading around the camp, including Sun. The two of them trek to the Staff and Juliet performs an ultrasound. The big, big moment of truth: who's the father of Sun's baby? Okay, so it might sound a little soap-operish on the surface but it's pulled off quite well in the context of LOST.
Elsewhere, Naomi's life is ultimately saved by an unlikely person — Mikhail "I Crush Your Sonic Fence" Bakunin. Even though he tries to beat Jin to a pulp and steal the satellite phone Naomi was carrying, Desmond lets him off the hook — because apparently "keeping your word" is more important than not jeopardizing the lives of your friends. I'm not bitter.
I have to give kudos to Kitsis and Horowitz; they are synonymous with LOST's comedic episodes, yet in D.O.C. they gave us a very emotional moment during Sun's ultrasound. It's just as important for writers to have a wide range as it is for actors, so hats off to them.

I should really wait until I'm completely done with the Redux rewatch, but as of right now (9/13/2010) I would have to vote D.O.C. as Yunjin Kim's finest acting. I'd even narrow it down to one scene: the ultrasound.
It's not just her tearful reaction to learning who the father was, it's the entire sequence beginning with entering the Staff. Sun is frightened for all sorts of reasons — she doesn't know what to think of the Staff, she doesn't know whether to trust Juliet, and she's about to learn whether her life is in danger. Yunjin plays ALL of this perfectly. Look at her face when Juliet rubs the cream on her belly and explains the process. She's a deer in the headlights, then marveling at the image of a beating heart. And then, yes, she breaks down crying for what appears to be one reason but is actually another. Amazing work by Yunjin Kim.
Naturally I must also mention Elizabeth Mitchell, who was equally important in making that ultrasound scene a memorable one. I personally feel there's a reason this scene was mirrored as a pivotal point in the series finale, and it had a lot to do with how well the actors pulled it off.
Daniel Dae Kim was far less prominent in this episode, but between his flashback with Sun and the physical acting on the island he deserves some bonus points. Gotta love the beatdown on Mikhail, particularly that nasty roundhouse kick!
Bonus points for John Shin (Jin's father), who had such a gentle and kind disposition in his scene with Yunjin. If I didn't know any better I'd swear he was indeed talking with his daughter-in-law.

This is the only episode directed by Fred Toye, and once again I'm amazed at how someone can slide right in and preserve the look and feel of LOST. Granted, this episode was dialogue-heavy and not the most challenging from a directorial standpoint, but you still had a few moments that could've suffered greatly had Toye been clueless.
Exhibit D: Jin's fight with Mikhail. A little too brief for my tastes but still wonderfully choreographed, directed, and edited.
Exhibit O: multiple chases through the woods. I don't care who's directing, this is likely never easy.
Exhibit C: Toye wisely keeps the angles close during the ultrasound scene, recognizing this was a big moment for Sun, not to mention a chance for Juliet to score some trust with the Losties and the audience.

I think D.O.C. holds up reasonably well but I'll concede one thing: if you're in the mood for mythology advancement you'll probably see Sun and Juliet's trek to the Staff as somewhat sluggish, especially on a rewatch.
The ultrasound scene is obviously significant in the Sun/Jin arc, but more importantly it stresses the urgency of getting Sun off the island in S4. The scenes with Naomi and Mikhail are of critical importance, paving the way for the freighter arc.

D.O.C. wasn't anything special in terms of atmosphere, however I'd have to be a robot to claim it didn't pack a hefty emotional punch. Normally I don't have much sympathy for a character who has committed adultery (male or female), but when Sun began the ultrasound process I might as well have been Jin sitting in the room with her. The fact that it's revisited in the finale just makes it all the more special to watch.
I'll also confess to having a bit of bloodlust in the final flashback, where Sun hands over the money to Jin's mother. I wanted more than just some stern warnings from Sun, I wanted some sort of retribution for her disgusting blackmail.
Judging from the high ratings in the other categories you might think I'd rate this as an amazing episode, but this is one of the very rare cases where I think the episode is LESS than the sum of its parts. The ultrasound scene is indeed amazing, and technically speaking the episode is quite strong, but D.O.C. is never an episode that immediately comes to mind when I think of mind-blowing chapters in the story that is LOST.