It's no shame in Pacquaio not being able to floor fighters like Margarito and De La Hoya. These guys had superb chins. I rate Mosley's power at WW and over two fights he couldn't dent De La Hoya's chin. It was very surprising what he did to Margarito, considering Margarito always had a reputation for having a granite chin. Most people who picked Mosley to win probably did so by decision. However, I still rate Margarito's chin. Pacquaio does have very decent power, but it's his workrate and accuracy combined with the decent power that he has which gets to his opponents in the end.
Margaritos conditioning along with his mental mindset having something to prove is what kept him in that fight. He took a lot of punishment, way more than he did against Mosley. Manny P is an excellent puncher with a lot of speed and power no denying that. His power is not overated.