Certain unifying characteristics describe quality runners. Whether you are a small and shifty runner like Ray Rice or a bigger power runner like Marshawn Lynch, you need certain skills to succeed on the field.
Vision to See Gaps
A running back with vision has the ability to be able to see the play develop on the field and anticipate when and where a hole in the line may appear. This skill goes hand in hand with agility and explosiveness, as a running back needs to not only see the hole but be able to burst through it to pick up yardage. Vision also allows you to know where your blocks are coming from and how to run behind those blocks to pick up as many yards as possible.
Speed is looked at a couple of different ways. If you are a smaller back, you need breakaway speed to break off big runs and outrace defenders. Larger backs also need the speed to get through the hole -- especially when you are near the goal line and trying to find your way into the end zone -- but slower running backs can make up for not having top-level speed with superior size and strength.
With many teams operating out of the shotgun, it is becoming increasingly important that running backs be able to catch the ball out of the backfield. While a player might be a running back in name, it's also important that he possess the ability to catch the ball consistently and pick up yardage via the pass; this makes a player a complete back. For example, Marshall Faulk was able to pass the 1,000-yard benchmark in both rushing and receiving yards at the peak of his career.
Fans may only notice when running backs score touchdowns, but scouts notice when they can block. At lower levels, running backs do not have to pick up blocks as much, but it is an essential skill in the professional ranks. Many times running backs have to pick up a blitzing linebacker, help on a dominant pass rusher or simply slow down a defensive back blitzing from the outside, but blocking is an important skill that good running backs need to have. Strength goes hand in hand with blocking, as running backs need to be able to slow down defenders who are bigger and stronger.
Another skill that running backs need to have is the ability to avoid the big hit. If you can be the enforcer and deliver some of the blow rather than absorbing all of the impact, you can stay healthier and avoid missing time due to injury. Durability is crucial for running backs, as some may handle the ball upwards of 30 times a game. Most of those plays end in getting tackled, so minimizing the damage on each hit is an ability that successful running backs need to have.