Keller's contribution is in the matchup problems he creates. It's not personified because of th lack of offensive firepower, but Keller has an ability to get open and is decent as an open field runner until first contact, where he is awful.
He often drops passes, is not good after first contact, and as mentioned by DD, has poor acceleration to get separation. His blocking is also not good.
He's a poor man's Hernandez. Not as fast or athletic. His contract should be less than AH's, which is not that high.
I feel that in Denver, NE, and offenses of that caliber, he could thrive, but right now he is Sanchez's security blanket, nothing more.