Kaplan died Friday of cancer in New York City, said his wife, Lisa Chase.
He edited the Observer from 1994 to 2009. The salmon-colored weekly has a reach beyond its circulation of about 50,000 because it is read by the Manhattan-based movers and shakers it covers.
Kaplan was credited with honing the paper's snarky tone and with hiring writers who became influential voices of their era.
Bushnell's column about love and dating inspired the hit HBO series "Sex and the City" starring Sarah Jessica Parker.
"The more cancellations we got for her column," Kaplan wrote in New York magazine in 2011, "the more the paper knew we had hit the jackpot."
Other writers who worked under Kaplan at the Observer include Joe Conason, who is now editor-in-chief of the political website The National Memo, and Nikki Finke, who founded Deadline.com.
After leaving the Observer, Kaplan was hired as the editorial creative director at Conde Nast Traveler. In 2010, he was named editorial director of the Fairchild Fashion Group, now Fairchild Fashion Media, a division of Conde Nast Publications.
But Kaplan's role as a cultural arbiter was perhaps best exemplified by the creation of several fake Twitter personae in his honor, including Wise Kaplan, Cranky Kaplan and Real Kaplan. Wise Kaplan's profile identifies him as "New York Oracle."
In addition to his wife, Kaplan is survived by their son, David; three children from his first marriage to Audrey Walker: Caroline Kaplan, Charles Kaplan and Peter Walker Kaplan; and two brothers, James and Robert.