What do you see as the difference between the 80's, 90's and current great QB's?

The differences between the QBs can largely be answered in the NFL rules governing WRs at the time. What Elway, Montana, Jim Kelly and Marino had to deal with is their WRs being mugged on every play and still were great. Today, Marino would throw for 100 TDs a year and Kelly would be in the 70 TD range. The p interference, personal foul penalties have changed the game significantly. I am not trying to take anything away for Manning or Brady because they have performed year in year out and both would have been great in the 1980s also.