I'm not talking about a writing flaw in your current project, more the flaw your main characters carry with them day to day (e.g. Monk - the TV series, it is his OCD which was also his greatest strength) or a personal obstacle they have to overcome (e.g. Luke Skywalker, his father is the enemy and he wants to sleep with his sister). Its the thing that hinders them, causes chaos/interest. My flaw is that the character is tough and ruthless, but completely helpless against the events/world around him. Whats your hook?