What is a fatal flaw?

A tragic flaw would be seen as something the protagonist is predisposed to, a flaw that colors the way he sees things or affects his actions. The character of Hamlet is predisposed to deep thought and contemplation. In essence, he thinks too much. Hamlet's decisons aren't made quickly, and the actions he contemplates are laced with responsibility. The need to make decisions that go against his values is difficult, and sometimes the delay contemplation causes changes the end result.