Football is a good skillset to come from if you are transitioning to MMA, but...
Football players (especially linemen), are used to running quick burst plays, and then having a frequent amount of resting time. Their "resting time" is a multiple of their active time. In MMA, you are fighting 5 minutes with 1 minute rest (and Pride's old rules of 10 minutes).
2) The quickness and speed required to land strikes and not be hit in return. A lineman can defend a guy that is lined up right in front of him, but remember those highlights where a faster guy (like Ahman Green, Larry Johnson, Preist Holmes, Jamal Lewis) gets some space between him and a lineman and BURNS him with a move. Point being: Standing RIGHT in front of them, they can tie up, take down, but outside of range, they are slow.
3) Angles in striking. This is something all combat fighters NEVER STOP LEARNING.
4) The BJJ techniques and the science behind what is a better position, what is available from positions, how to get out of positions, how to transition.
Im not saying a lineman CAN'T make a successful career in MMA, but they DO have a tough hill to climb to adapt themselves.