You guys may not agree with me but i think cutting promos at the level of the likes oh rock austin y2j, is just something you cannot learn i think your born with it, same as in a way with in-ring ablitiy yes you can teach them the basics or evan a few classy moves but cena is never going to be a GREAT wrestler same as hogan never was imo. the best promo guys are just being themselves but with the volume turned way up and beliving what they say on the most part.
I feel I should add to this that no amount of natural talent alone will make you The Rock on the mic. He worked damn hard on it too.
It's hard to say what makes him so good. While normally I'm pretty much a smart (despite my better instincts not to be) The Rock makes me absolutely mark out every time he grabs a mic.
I think the biggest thing he has going for him is a real instinctive understanding of how to read the crowd. He can hear in what they cheer and how they cheer and how loudly they cheer EXACTLY what they want to hear, and then deliver it. (He doesn't do it consciously, I don't think - CM Punk is pretty good at this too, but I can't help feel he is quite deliberate about it, while Dwayne Johnson would probably describe himself as going with the flow, or something akin to that.)
The charisma and confidence were a huge help for him. He could make big, outlandish statements with enough conviction that you'd believe he'd actually take that announcer table, polish it, shine it, spit on it for good measure, turn it sideways and stick it up your candy ass! Whenever he promised to Layeth the Smackdown he was (usually) able to back up his talk with his ability, adding to his mystique.
Finally, The Rock was a silly, over-the-top cartoon character in quite a grunged out WWF. He stuck out like that.
I have to agree with Uncle Chester's final point too. Foley, Austin, Flair (never saw Piper) all had superb - if more understated - mic skills. Foley in particular was a natural storyteller who could really get you behind or against his character in far more subtle way than Johnson ever managed.