The beauty of football is how it evolves from year to year, with new schemes and trends always emerging. No position reflects this better than safety.
Most programs used to recruit big physical safeties that would intimidate receivers over the middle and be especially effective in the run game. These guys were almost like having another linebacker on the field.
However, with the ever-increasing use of multiple-receiver sets in today's college game, this type of defensive safety became a liability in passing situations. As a result, the qualities of a defensive safety have come full circle, and now programs are looking for more athletic, "range"-type players.
They still must be able to hit and come up against the run, but their ability to break down and tackle in the open field is very important. Many guys can create a collision that looks good, but if they can't break down and make a key tackle in the open field, they will hurt a defense.
These players must have range and at least have enough cover ability to match up on the slot receiver if the nickel package isn't on the field or be able to cover a back out of the backfield -- not an easy task.
The strong safety must be a run stopper with the ability to at least cover the tight end while the free safety is more of a range-, centerfield-type guy with good instincts. They also make the secondary calls and put everybody in position.