Christina Hendricks (Joan) gets lots of attention for her Monroe-/Jayne Mansfield-esque figure, but to me she's principally an amazing facial actress - Gong Li, Maggie Cheung, Stephane Audran are good reference points for her. Elizabeth Moss (Peggy) acts in a more balanced way, through her face and body and limbs, but opposite Hendricks her facial acting blossoms.
At any rate, in this crucial payoff scene, where the characters really spoke for the shocked audience as well as for themselves, Hendricks and Moss were on fire, bouncing off each other, their eyelines so exact and locked in, their mutually translucent skin making every other face on the show look like a mask, and so on. Related bouquets: the precision camera-work and editing captured everything with minimal fuss; the lighting people performed their usual miracle of making an ostensibly fluorescent-lit, totally interior room seem realistically lit yet still warm and flattering to faces and complexions; shrewdly chosen costumes - Joan in black and Peggy in dark-gray - made them business-like and adult; and thank god for cigarettes. (Cigarettes are so so good for visuals and acting, pointing scenes. I mean, just look at the images above.)
The upshot: in some respects, and regardless of what Matt Weiner wants or thinks or plans, Mad Men is Peggy and Joan's show now. And if it isn't? Well, we'd pay money to watch a post-Mad Men, rise and rise of Olsen Holloway in the '70s show.
Jan. 2011 update: Fabulous interview with Elizabeth Moss here.