Big deal? yes. Lining up a foot inside or outside of the right spot puts a player at a disadvantage from a "positioning" standpoint. Winning very play isn't a bench press contest or a 40 yard dash - it's putting yourself in a position to defense or "take away" what the offense is trying to accomplish. When a defensive player is in the "right" position, the odds of the offense being successful go way down. Transversely, when a player lines up incorrectly, he loses position or leverage, and takes himself out of the play. Good defense, just like good offense creates mismatches - the defense accomplishes it with positioning and numbers though.
Reading the Offense and Communicating
On any given play a lot goes on "pre-snap". There's a constant cat and mouse game between the offense and the defense. The offense trys to confuse the defense, and vice versa. The next time you're at a game watch the defense prior to the snap on any given play - the Linebackers and Safeties are usually pointing and yelling signals to the rest of the defense. A simplistic example would be "Strong Right" - a call made by a Linebacker which means the Tight End has lined up on the left side of the offensive formation. The Linebackers and Safeties are usually able to see the entire field from where they line up so they do the reading and instructing. On any given play both space, and each offensive skill player must be accounted for.
Dictating the Offense
After fundamentals are addressed correctly on each play the mental gamesmanship takes over. You've heard, "A good defense can keep the offense guessing all game long." That works both ways of course, but disguising defensive fronts effectively can confuse the offense's pre-snap reads. This allows a defense to pressure the QB and shut down the run, by creating mismatches with numbers. Creating mismatches with numbers just means you're able to put more defensive personnel in a strategic area than the offense has blockers.
Recruiting and Personnel
You hear the expression, "when a coach gets his players in the system he'll have a chance to win." Not all players are a good fit for a defensive formation. It's up to a coaching staff to find the right players for the system or formations he will utilize. An example on defense would be a prototype 3 man front Defensive End. That player is usually much larger and often slower than a 4 man front Defensive End. The reason is that in a 3-man Front pressure comes from the Linebackers, the Defensive End in that formation is asked to mostly defend an area or "gap" and tie up blockers to allow the Linebackers to make plays. In a 4-man Front the Ends are smaller and faster, they are expected to provide pressure themselves. Think of them as oversized Linebackers in terms of quickness. Putting players in the right position to utilize their abilities is a fundamental of coaching. Sometimes player types are found through recruiting, sometimes they are developed once they get on campus. Regardless, proper personnel for a system is critical.
I've given just a few components of any good defense (there is much more to it - mental approach being a big one), now I'll spotlight the formations for discussion sake:
3-4 - base alignment uses 3 Defensive Linemen including a Noseguard, 4 Linebackers. Must have a big 2 gap player at Noseguard. Pressure often comes from Outside Linebacker. 2 Safeties, 2 Cornerbacks.
4-3 - base alignment uses 4 Defensive Linemen, 3 Linebackers. Must have a good Middle Linebacker and fast Defensive Ends. Probably the toughest defense to recruit for. 2 Safeties, 2 Cornerbacks.
4-2-5 - base alignment uses 4 Defensive Linemen, but only 2 Linebackers. Linebackers have to be able to cover a lot of ground. Employs 3 Safeties, which are interchangeable at the highest level. 2 Cornerbacks.
3-3-5 - base alignment 3 Defensive Linemen, 3 Linebackers. Teams that run this often "stack" Linebackers behind down linemen, then vary angles of pursuit on every play to confuse blockers. Also uses 3 Safeties. 2 Cornerbacks.
There are several more formations such as the 5-2 and 4-4, but these aren't seen a lot at the collegiate level.
Now it's your turn "Coach" - come inside the Owl's Nest and tell me about your favorite defense...