-the costumes (sooo feminine! when considering the female characters that is).
-the misogynistic attitudes (even women think that typewriters are normally too complicated for a woman to use, but *thank God*, men made women a favour and made them fairly manageable for them).
-the slight racism (African-Americans can only be cleaners and waiters).
-the jazz music (fly me to the moon, let me play among the stars, let me see what spring is like on Jupiter and Mars...).
-the NON-STOP smoking (it's amazing how they haven't all died from lung cancer yet).
-the character of Peggy, the ugly duckling who somehow still keeps her cool with all the [insert rude word] male colleagues she has.
-Betty's costumes (covered in the first point, but I just had to mention Betty separately - oh those dresses).
-the amazing, AMAZING character of Joan, with the red lips, the curvy figure, the confidence, and the charm, so 1960s! (oh wait, the whole series is set in 1960s Manhattan).
I can assure you this list could have been very long. Maybe it's because I have a thing with the '60s.
I'll leave you with a bit of jazz :)