1) Short Yardage and Goal Line - Surprisingly Jackson struggled on 3rd and 1 and 4th and 1. He had a hard time getting his body going. Gurley has a way on making himself skinny and finding the end zone.
2) Burst, Gurley in the 4 first games and the ability once he was in the 2nd tier to burst out and go.
3) Lastly, as you eluded to, elusiveness. The threat of a big play is so significant in the NFL.
I do think Jackson was a good receiver, but we don't know what Gurley could become because of The Fisher Offense being so limited.
The thing I thought Jackson was so good at that very few backs had was catching that screen pass in the flat with space to get his body going. When that happened, he was a dangerous player and could deliver punishment to the opponent.
I feel Gurley could still be a once in a generation player. I never even felt like Jackson was a top 5 back in most seasons.