Injury history.  Look at how often the player has left games early or lost effectiveness as the season has worn on.  Does the player deliver the hit, or does he get hit?

Look for good decisions about when to change hands, such as shifting to the opposite side from an anticipated hit.  When shifting ball position, player should maintain security, and when the ball is tucked away, all pressure points should be covered (ends of the football, though for some players, this may require two hands to additionally cover two sides of the football as well.  This is related to the size of the player).  When taking hit or being tackled, player should get two hands on the ball, and quickly.

This difficult-to-measure ability requires that as a viewer, you know where the play is designed to go.  Once you know that, look to see how quickly the player makes a decision to commit to an angle and running lane.

Additionally, when the player has committed to a running lane, note the direction of the blocking angles around him, and then note how his angle matches up with his blockers.  The best way to think about this may be to look for "flow."  Watching a play from on high, you can see the general direction of the play "flowing" in a  particular direction.  A runner who matches this flow when he chooses his running angle and lane will get extra yards just from the momentum of the play itself.

An important note here is in the analysis of one-cut running schemes, because they teach the opposite:  when the flow goes a certain way, the RB needs to choose a proper cutback lane, which makes him go in the opposite direction of the general flow.  While this method is generally superior at creating open running lanes, it lacks the extra yardage of a "flow" system, and it requires a RB who is able to make second-level defenders miss, or who can bend an angle to such a degree that the defenders are unable to compensate.

While toughness is a primary concern, look also for loose hips and an ability to swivel in tight spaces.  Quickness off the snap is important, and decisive-cutting ability.  The propensity to fall forward and gain yards is an important factor a back.

Straight-line speed is the easy part of this equation, as it requires a back with strong cutting ability and a good vision for taking proper angles. This is a measure of a player's ability to get to the edge and turn the corner.

Generally a measure of lateral shiftiness, it can also measure a back who makes himself small going through a hole.  Some backs base this part of their game on "moves" such as head fakes, stiff-arms, or spins.  Coupled with proper ball security, these moves can be extremely effective.

Look for good forward lean, low shoulder pads.  Legs should have a lot of drive and keep moving, and back should fall forward when tackled.  On contact, back needs to be able to maintain balance, and look for backs who break tackles and reestablish momentum after contact.

Note blocking-angles chosen specifically, and note performance on chop-blocks, lead-blocks and kick-out blocks.  Smaller backs should be able to attack the hips and thighs without losing awareness of the defender or losing balance, while larger backs should be physical and use leg drive to engage and redirect defenders.

Does the back only give halfhearted effort?  Watch out for backs who "block and watch," where they throw a block and then watch the defender instead of finding the next target.  Does the back make the best blocking choice for his body type?  If he doesn't, could be a sign that he isn't listening to his coaches, or doesn't care about that aspect of his game..

A measure of the balance and quickness a player exhibits coming out of his stance and approaching the line of scrimmage.  You are looking for consistency most of the time, and in certain formations like goal-line or short-yardage you are looking for explosiveness and quickness.  In general, players should grade out adequately here, though occasionally you might note the surprising quickness of a larger player.

Does he catch with his hands?  Is he able to catch outside his body?  Does he look comfortable reaching out to get the ball or does he "double-catch" or bobble the ball?

Are his routes out of the backfield well-timed?  He should be coming open as the QB reaches his final check-down.  Can he come off a block and be ready to catch a pass?

Does he use his hands to maintain separation from the defender?  Does he move well laterally when choosing his blocks?  How well does he pick up the blitz, and does he understand where pressure comes from pre-snap?  Does he sustain his blocks?  Look for a back who can choose proper angles and times the blocks well to avoid "high-low" penalties.