Austin: Despite kicking over old ladies got pops everytime he came out.
Rock: Had to "sell-out" the business to get a 50% heel reaction
Hogan: Had to wish death on every WCW fan and wrestler to get a midcard heel reaction.
Brock has IN-RING charisma. the kind that allows you to connect with the fans and tell a story to them, and the fans respond. he also does a great "serious" interview (See: face-to-face with the Undertaker) which everyone says Benoit does so well. (And he does.)
The fans cheer Brock cause they want to see him get the best of Heyman who they hate.
His promos have never been good in my eyes even his ones with Taker. He just doesn't sound genuine when he says it.