The changes to defense are directly related to complaints that NCAA players have had for quite sometime (i.e. DB's creeping out of nowhere to snatch picks and LB's swatting down balls without even looking). Now the defensive player must be aware of the ball's location before making a play on it.
AI has also been improved to the point where it is not so easy to burn DB's any longer with a speedy receiver. The developers have added techniques like trailing and playing off-man, making the game way more realistic.