
Here we have another solid episode from Elizabeth Sarnoff and Christina M. Kim, the same team who penned The Hunting Party and Two For The Road (two killer episodes which aired shortly before and after this one).
There isn't a strongly defined, shared theme between the two storylines but it's not really necessary this time around. In flashbacks, Jin and Sun discover they have fertility issues, which further drives a wedge into their already tumultuous marriage. If that weren't enough, Sun has become fluent in English thanks to her tutor (and maybe more?) Jae Lee. There's an awful lot of apparent chemistry between Sun and Jae, leading us to think that the "whole truth" may be elusive this time around.
The big reveal on the island: Sun's pregnant. Initially I thought, "Great, now we can have two characters running around screaming about their baby!" but I should've given the writers a little more credit. Anyway, as Sun breaks the news to Jin (in the garden, where they seem to spend the majority of the episode) she insists she's never been with another man. Bear in mind that while her lips say one thing her eyes say about thirteen other things.
Henry Gale draws a map to his balloon for Ana Lucia, who leads Sayid and Charlie on Jungle Trek #33. Interestingly enough, Sarnoff and Kim admitted in the episode commentary that it was a real challenge to build sympathy for Ana Lucia. This would explain her uncharacteristic vulnerability in the campfire chat with Sayid. She comes right out and admits, "People don't like me."
Personally the play for sympathy didn't work for me, but perhaps I'm in the minority.
Just as the Jungle Trek gang reach the area where the balloon's supposed to be, we see Jack bring Henry Gale to the dining nook for breakfast. This, of course, leads to one of the most memorable monologues by Michael Emerson. Much credit goes to Sarnoff and Kim for the writing here, for not many cliffhangers get much better than that.

Even though this isn't my all-time favorite Jin/Sun-centric, it's probably the best acting performance by both Kims. This is a really challenging episode in terms of scaling the entire emotional spectrum — sometimes multiple times within the same scene!
Yunjin Kim and Daniel Dae Kim had really perfected their on-screen chemistry by now, covering much more ground with their facial expressions than scripted dialogue could ever convey. They pull off at least three different arguments — one in the bedroom, one in the garden, and one in the doctor's office. When you compare that to Jin's elation post-announcement and that should illustrate what a masterful job both of them did here.
If Michael Emerson hadn't already secured his place on the series for the long haul, The Whole Truth made it an absolute certainty. His scene with Ana Lucia in the armory is a clinic on the subtleties of acting. Naturally his "got any milk?" monologue is one of the pillars of LOST. Many kudos to Matthew Fox and Terry O'Quinn as well, for even as Henry Gale was waxing poetic their reactions spoke volumes.

This is one of only two directorial efforts by Karen Gaviola, which is a little surprising because The Whole Truth feels very much like a standard LOST episode in terms of pacing and visuals.
By far her best work is in the Henry Gale monologue at the very end. The shots so perfectly capture the building intensity of the moment, telling three different stories as we switch from Gale crunching on cereal to Locke raising his eyebrow to Jack staring incredulously.
Gaviola also plays with us a bit in the scenes between Sun and Jae Lee, opting for a wide shot after we were convinced they were closing in for the kiss. I'm pretty sure the audience was supposed to feel conflicted about the possibility of an affair and Gaviola took that game one step further.

This episode's got quite a bit of casting variety which really helps with the replay value. You've got the obvious teams of Jin and Sun, the Balloon Crew, and the guys at the Hatch, but then you've also got Rose and Bernard, a sprinkling of Sawyer here and there, and even a short visit from Hurley.
In terms of significance, this episode certainly sets up a lot more Jin/Sun material in Season 3. But I can't necessarily credit The Whole Truth for being the critical installment of the Henry Gale arc, because he still hasn't offered that definitive jaw-dropping moment yet (it's coming).

On the episode commentary Daniel Dae Kim suggested he could detect some sexual tension in the scenes between Sun and Jae Lee. I think that might be the understatement of the decade and I'm pretty sure "The Whole Truth" is an intentionally ironic title.
If Charlie hadn't already set a precedent in "Fire + Water" of lying straight-faced to someone, I might have believed Sun's insistence that she hadn't been with another man. But even after her warm embrace with Jin I wasn't convinced, and I suppose that's the point after all.
Oh, there's some emotional impact in the episode — unfortunately it all comes flooding in as the "L O S T" title flashes on the screen. Henry Gale's monologue is undoubtedly a turning point in the Season 2 narrative, one that tells the audience that we're in the home stretch.