So Poppy's only in town for about 48 hours, and basically they play out every conversation between Whitney and the perfectly perfect Olivia on The City, sans switchblade. (I honestly do just adore Olivia, which you probably could have guessed if you've seen the show; she's the reason I watch it, because she's this monstrous Cassandra who is always, always right, but you can't even handle how right she is because you are blinded by the inhuman force-of-nature way she does everything. She's like if Simon Cowell were the prettiest girl in the whole world.) There's a beautiful crest where the scheming music of this show kicks in just as Serena's realizing that all these cues she's expecting -- "Oh, you broke up? That's so sad!" -- are not forthcoming, because Poppy couldn't care less. "Wow. Well, it sounds like everything's exactly the same as it was last time I saw you..."
Going crazy for approval, Serena says she's throwing a party this weekend, cocking her head at Poppy needily, but them admits it's for her friend's Sweet Sixteen. Poppy mentally crosses Serena off of every list there is in the world, and Serena half-heartedly invites her, begging for the conversation to end and knowing she'll never see Poppy again. And knowing too that she'd do the same, because it's the right thing to do. Poppy distractedly tells her to have fun with her "little party" and "take care" of herself, and pats her hand, and entire parts of Serena's soul go skittering off into the road, and it's over. But consider how what Poppy just did was the equivalent of kissing Serena's forehead -- like Poppy's a woman and Serena's just a little girl -- and you see how this whole episode works.
And not just that, but the Yale stuff too, because it was Poppy Lifton that got Serena on Page Six, which got her into Yale if she'd wanted it, but which she turned down because she didn't earn it, and to save Blair, and neither of those things really worked out either. So even though we haven't seen Poppy in a while, she's been here, implicit every time anybody mentions Dean Berube or that damned press release. I would say that what separates the Great episode of this show, or most shows really, from the merely Good ones is the fact that everyone is real: nobody's serving as a prop in anybody else's scene. Like how Dan was often more of an irritant or plot factor for Serena's stuff, in the first season, than a character in his own right, or how Rachel Carr was only ever a mirror for the person -- Serena, then Dan, then Blair -- that she was molesting.