The event was an evening wedding at a nice hotel in D.C. (black tie stipulated on invitation), with ~250 guests (no children invited) who were mostly professionals and covered a very broad age spectrum (20-80 years old).
Almost all men wore black, but just as many wore black suits as proper dinner jackets. Of those wearing dinner jackets, the vast majority had something 'wrong' per the rules circulated on the forums: lots of two- or three-button single-breasted jackets, more notch lapels than peak lapels, vents almost universally present. More esoteric options were almost entirely absent. One other person other than myself wore a shawl lapeled jacket.
Shirts were mostly white, regularly buttoning business shirts. I only noticed two or three men in studs, and they were all older gentlemen. All studs I saw were simple onyx on gold or silver. I noticed no other guests wearing single cuffs. However, I also noticed no barrel cuffs, either. Everyone wore French cuffs.
Cummerbunds and vests were rare. Maybe a third or fourth of the men there wore bow ties. The clear majority wore long, black ties. A surprising number wore pocket squares, but not well--lots of folded white silk.
Shoes were a random assortment. It appeared as if men simply picked whatever black business shoes they happen to own. A lot of ugly square-toed loafers. Lots of cap toes, brogues and derbies. I saw maybe four or five other men in patent leather. No evening slippers to be seen.
There seemed to be no discernible norm for women. Some wore short cocktail dresses (as my wife did) and some wore long, floor-length gowns. Some even wore dressy blouses with short skirts. Generally the dress length correlated with age. Many had the good sense to wear some sort of shawl during the ceremony (which was very religious), but I was surprised by the number that remained bare shouldered. At the past few non-black tie weddings I've been to, shawls were universally worn during the ceremony.