Speed kills, and unfortunately, much like the wide receiver position, the evaluation of the running back position at the high school level is often entirely based on speed.
However, qualities such as quickness through the hole, run vision, pick and slide, good balance, and change of direction skills are every bit as important. A player termed an "all-purpose back" with excellent size and power is the ideal prospect for most programs.
While speed is nice, quickness is great. There are a lot of great running backs at the college level that lack great 40 speed, but have exceptional quickness.
Also, with so many spread offensive sets, today's running backs must be versatile enough to have run skills and catch the ball out of the backfield. The running back with the ability to split out wide and create matchups in the passing game is the ideal choice. It also helps if they have some skills as return specialists.
There are plenty of other areas used to judge running backs. The ability to make tacklers miss and eliminate a lot of head-on collisions is a great indicator of a back's ability to stay healthy.
Blocking is another under-evaluated skill when it comes to grading backs. Most freshmen entering college have not spent a lot of time working on their blocking skills. If you ask any college coach, a freshman running back is more likely to sit on the bench or come out on passing downs not for his inefficiencies as a runner, but for his lack of success as a blocker and his inability to pick up the blitz.
It is very rare when you get a back who can do everything: run, catch and even return. These guys give opposing defenses nightmares, and they can change the personality of an offense. The Marshall Faulks of the world are rare, but their all-around ability and versatility bring a different dimension to any offensive philosophy, and that's what every recruiter in the country is looking for.