User blog comment:Yatanogarasu/Gecko Moria: Weakest of the Eight?/@comment-910220-20100701055802

"Moria was the only one of the seven Shichibukai present at the war to display shock at Luffy being Revolutionary Dragon's son, an indication of lack of composure."

Okay, here's the explanation: the other Shichibukai managed to take this news without freaking out, but Moria didn't. This means he is easily scared, a sign of weakness.