I, personally, would consider the fact that Hamlet is a thinker rather than a doer to be his most important trait. Throughout the work, he is contrasted with Fortinbras of Norway, who is a doer - ready and willing to fight for his country, to do what is necessary to get the job done. Hamlet, by contrast, considers and ponders things a great deal. When given the chance to avenge his father's murder, he hesitates and thinks about whether or not Claudius' soul will go to heaven if he is murdered while praying. There are several instances of his unwillingness to act quickly when the time is ripe, and this ultimately leads to his downfall.
Check the link below for more information from eNotes. Good luck!
Hamlet's most significant trait is his indecision. It relates to the larger work as it is his hamartia.