SI.com just released their list of the Top 25 most hated teams of all-time. The 1974-1975 Flyers came in at number four.
As referenced by this cab driver, who still has a lingering hate for Bobby Clarke, the Broad Street Bullies were despised in the hockey world. The recent HBO documentary did a good job of explaining this, quoting one writer who said, "they represented everything evil about the game."
Of course, winning two Stanley Cups doesn't hurt either.