I saw two threads over there concerning this subject. One was a "Once in a generation for your team" thread and the other was for the whole SEC.
I only read the first two pages of the "whole SEC" one, but it looked pretty unanimous that Cam was the only choice with some arguing that Walker (legit) and Manziel (also legit) were worthy candidates.
The other thread had each team arguing who their top one or two players were. Eli Manning and Patrick Willis would both be once in a generation type talent for Ole Miss. No argument there.
For me, I see "once in a generation" as a player that is heads above everyone else in talent and skill in a way that evolves the position. Pretty much, every position could have a once in a generation player and some positions would have multiple players.
For quarterback, Cam Newton is a once in a generation player because he's the first quarterback to ever both run and pass in the manner that he did. Not only was he elusive, fast, and accurate, he was one of the largest quarterbacks to ever play the game giving him a size advantage over every defensive player except defensive tackles and run stopping defensive ends.
But he wasn't chaotic or wild in his play. He played the read option very well. He drove the line of scrimmage. He threw the ball with good timing. Where Cam Newton was precise, Johnny Manziel was innovative.
So now I'd say Johnny Manziel was a once in a generation type of talent. When's the last time we've seen a quarterback that can perform snowboarding stunts prior to throwing a 30 yard pass accurately? Name another quarterback that can elude pass rushers in a quickly collapsing pocket, juke a linebacker, then outrun the fast cornerback? Manziel is a once in a generation player.
You could say the same about running backs. Bo Jackson was a once in a generation talent. Sheer size, power, and speed. He was unstoppable. I'd say that a guy like Reggie Bush - though not in the SEC - was a once in a generation talent at running back because he can catch, return punts, kickoffs, and juke just about anybody. So Bo and Reggie are completely different players with different skill sets and are both in the vicinity of a generation, but they are both once in a generation talent at running back.