The quarterback is the leader of the team.  He calls the plays and takes the snaps and after every time he takes the snap, he has to make sure he does the handoff right, doesn't take and extra step on his drop back, or doesn't overthrow his wide receiver.  He could be honored as the best player on the field, or the worst.
The quarterback has to throw the long ball with a right on lob ability or a short pass with perfect accuracy.
The Running Back is another vital part of a working successful offense.  They have to make sure they don't fumble the ball while trying to run through near 6'3, 300 pound

lineman , 4 linebackers and 4 defensive backs.  That's tough work considering most running backs are only about 5'10, 210 pounds.  They also have to be able to turn on the turbo while turning a corner around the end of the line trying to make a mad dash to the end zone.  They should be able to juke, spin, stiff-arm, and jump over defenders with ease.  They should be able to run around a 4.75 to 4.9 40 yard dash on any day.

The tight end isn't a vital position on every play but it plays a big part.  The Tight end should be able to catch passes and run for big gains, or be able to block at the line like a lineman.  Most tight end's could also pass as a Fullback, Tailback or even sometimes a Wide Receiver.

The wide receiver position is a pretty large part in most strong offenses.  The wide receiver is usually the tallest player on the team.  They also are the people who can run really, really fast and jump very high.  If you are a wide receiver you should be able to catch almost anything.
The offensive line is the backbone of the whole offense.  If you don't have a good offensive line you won't be able to score.  With out them there would be no touchdowns, or no glory for any other player.  There'd be no big gains nice plays.  They are the main part.
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