Most important positions?
1) QB - touches the ball every offensive play and has to make correct decisions. Hardest to find.
2) DT - determines your ability to stop the inside run AND whether you can rush 4 or need to bring more
3) DE - Need pressure
4) OT - Need to stop the pressure
5) IOL - Determines whether you run inside or out, and whether you use a quick passing game or work verticals for big plays
6) CB - need to stop the WR
7) WR - need to beat the CB
8) LB - lunch pail guys who need to stop the run and be able to cover
9) S - the make or break a D guys.
10) TE - if they can block AND are dynamic in the passing game they are a huge asset to the offense
11) RB -- shortest life cycle in the game. Lots of backs can run through huge holes, but few can run if there aren't any holes at all. They are usually small by NFL standards because they need their quickness and speed, but end up blocking guys lots bigger quite often in a game. Then they get hit by guys with lots of momentum.
The name of the game on defense is to make the other team one dimensional and then stop their ability to do that efficiently.
The name of the game on offense is to do what you want when by confusing the defense, or dominating the line physically, or finding the defenses weakness and exploiting it.
The game hasn't changed in 50 years, but seems like it is all new. Really coaches just have different teams with different strengths, and learn to exploit teams in differing ways. It runs in cycles as to who the ultra elite are, but the game really hasn't changed much. Only real changes I have seen are the skirts on the QBs, and how blatantly the officiating controls what a team can and can't do during the game.