I was talking to someone about Roger Elwood, some time in the last few weeks, only I couldn’t remember his name. Elwood . . .

. . . is best known for the bizarre episode in which he flooded the SF market in 1972-1975 with carelessly edited theme anthologies.

Prior to that time, anthologies and collections were very popular with readers, and were considered by the publishing industry to be a surer bet than novels. Roger Elwood ended that, singlehandedly breaking the story collection / anthology market. It has never wholly recovered. He squandered industry credibility accumulated over decades by better anthologists, and wrecked the readers’ faith in collections. . . .

By the time Roger Elwood was finished, you couldn't have sold an SF anthology into the North American market if it were priced at ten cents and made out of Godiva chocolate.

Wikipedia has the scoop. With graphs and everything.