I think the biggest difference between "normal" football fans and the football enthusiasts like me and the ones I met in my engineering classes boils down to what reasons you enjoy watching the game and, perhaps with overlap between the groups, what level you're paying attention to the actual plays being called and run.
Most people love football because they identify with a team, and almost exclusively watch games involving their team, and aren't really paying too much attention to the strategy. They know the rules and the different types of plays, but they are really there for the tribal passion that a rough contact sport like football tends to evoke.
On the flip side, I don't actively seek to watch football games for a single team; if I happen to go somewhere that's playing a football game on TV, I love watching it, and don't care who wins, because I just enjoy the strategic and tactical decisions being made on and off the field. Watching a play unfold and seeing the snap decisions being made and the reactions from both teams to the changing field is truly fascinating.
I do still follow the results of my old hometown team, the Cincinnati Bengals, but more out of humorous pity at how terrible they are year after year than out of any identification with them or desire to see them win.
But hey, maybe I'm just ascribing what I want to see from the data, and maybe my opinion of "normal" people is skewed from growing up in Ohio compared to somewhere else in the country.