Greeley's published output was so immense (120 books? 150 books? The priest who gave the eulogy at my father-in-law's funeral had composed two operas about Chicago politics -- not surprisingly, his librettist was Greeley), that it's hard to get a grip on the essence of his contributions. The NYT obituary, for example, concentrates on his steamy novels and battles with Catholic Church officials (Greeley denounced the "lavender mafia" within the Catholic hierarchy, but of course that doesn't get mentioned in the obituary) rather than his social science side.
I would call Greeley the intellectual spokesman for the white Catholic urban ethnics who got hammered by integration and the Great Society (Greeley was born in the same Austin neighborhood of Chicago as my wife), the people who saw up close and personal early on what welfare was doing to blacks. But, who wants to remember that?