Wonderful article, PP! I'm not a fashion expert. So, for me, the most striking thing about GF's costume design is that it reflects FFC's fanatical attention to detail. In Connie's wedding, you see that the men are all wearing short ties because of the wartime shortage of silk and synthetics like rayon. But look closely and you'll see that the young girls' hair is tied with strands of wool, not ribbons, for the same reason. Another nice touch: the old guy who sings puts his glasses in his suit jacket breast pocket: he's too vain to wear them while performing, but wants to remember where he put them.
Kay's picture hat is right on target for the period, too. Fredo's foppish outfit in Vegas was "high roller modern" for the period (as well as a dead giveaway to his impending treachery).
The only thing I thought was a bit overdone was Michael's outfit when he woos Kay in New Hampshire. With that ridiculous homburg hat, he looks more like a pint-sized banker than a "legitimate" businessman on a lovelorn mission.
David Thomsen, the famous UK film critic, has written that Pacino's style as Michael totally transformed the image of gangsters from what Mark described as the cliched stereotype.