To give you a little history about broken backboards in the NBA, Chuck Connors is credited with the first, although not through a dunk, but with a bricked shot during warmups (there was a failure to attach a safeguard by one of the court personnel that would have prevented this from happening). As far as dunking to break rims though, Daryl Dawkins is credited with the implementation of the "breakaway rim" after shattering several backboards with dunks, also resulting in a rule being created fining anyone that breaks a backboard.
What made Shaq's dunks in NBA games so special though was that it really wasn't just the backboard being broken. When Shaq dunked, he put so much force into it that he was physically taking down the entire structure. It's so impressive that it wouldn't have been a shock to see him turn green and begin losing the eloquence we're used to hearing him speak with.