The qualities in a defensive end vary with the type of defense a program plays and what the team asks a player to do.
Traditionally in the 4-3 look, teams are looking for defensive ends who can rush the quarterback. The weak-side defensive end should be the best pass rusher, and he must have speed and explosion off the corner and should be a potential sack artist. A good weak-side defensive end should have great range and chase ability and at least be adequate against the run.
A defensive end who struggles against the run will be exploited by opposing offenses who find him and run right at him. Many freshmen coming into college football lacking great size and strength become successful contributors as weak-side defensive ends, either on a full-time basis or on a limited basis on passing downs.
The strong-side defensive end in a 4-3 defense still has to have some pass rush ability, but is also a little more powerful and stout. He must be able to stop the run (most teams are right-handed), and he must be able to hold up the tight end at the line of scrimmage. He is not quite the athlete that the weak-side defensive end is, but he is more capable of anchoring. The perfect kid has enough size to stop the run, but also the quickness and athletic ability to put pressure on opposing quarterbacks in the pass rush.
In 3-4 defenses, the defensive ends are more physical and are asked to play the run more than pass. In this defense, the primary pass rush comes from the outside linebackers, and the defensive ends are asked to stuff the run and keep the blockers occupied. These players are usually tough guys with size and some athletic ability, but not necessarily proficient pass rushers.
Obviously, programs are going to recruit to their respective team philosophies and since most 17-year-old kids are underdeveloped at this stage, it may take some time to figure out which side they will end up playing.