#1 His ability to build a program
One of the best qualities about Joe is his ability to help create a program and be actively engaged in its delivery, throughout his career. Fans would be hard-pressed to think of a moment when Joe wasn't there to either complete a heel turn, or lead a faction, or simply build towards a one-on-one battle either as champion or challenger.
In comparison, Brock can't do that. Whether we attribute that to his schedule, or a dependence on Paul Heyman to deliver endorsements of his client, his direct engagement in his storylines is more limited. While there's nothing wrong with having someone speak on your behalf, it most certainly doesn't hold as much value if it isn't coming from the competitor.
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Joe hasn't had someone speak on his behalf. He has had allies, or walked alone, but has always been the one to lay down the challenge or stand up to the challenge, either in promos or vignettes leading up to an event. Joe's consistent attendance and promos have made his ability to build a program that much more impactful.