He's "heavy handed", but so are most guys. I don't think it's STRICTLY a matter of power, nor do I think he's quite the KO artist people make him out to be. The Bisping KO was a combination of perfect timing, Hendo selling out and putting his whole body into a very accurate punch, and Bisping walking right into it.
The real difference is that Henderson is willing to load up and throw that big haymaker where most fighters will not due to the fear of being countered. The punch that landed on Bisping would KO anybody, and in theory, could be thrown by nearly every other 205+ fighter with similar results. The willingness to fire it is what sets Henderson apart. In comparison, I'd use Rashad Evans, who is NOT known for heavy hands, but was able to land a similar KO against Chuck.
Not taking anything away from Henderson with that. I just think it's more accuracy and the will to do it rather than actual power. When I think of pure KO power, I'm more impressed with Rampage/Wand3 and Chuck/Couture.