One thing I look at is "tough yards", did the back get YAC, could he avoid tackles, or run through them, could he be relied on to get two yards consistently.
Herschel had of course a lot of "highlight reels" where the defense broke down or he ran over some 190 lb safety. He also averaged 35 carries per game his second year, which is rather astounding today. But a good defense could stone him at the line and shut him down some.