Picking up on something Main Man said in the "Did Gertrude Know?" thread, let me ask a followup question:
What do you think Hamlet's relationship was with his father?
I bring this up because in one response MM asks, "why is Hamlet at Wittenberg if he is so close to his father (particularly at 30)? Could it be that Hamlet's paternity could be in doubt?" and then later he states, "He does want Hamlet to avenge "his death," but I do not think they have a good relationship."
I happen to agree. I've always thought Hamlet's father to be something of a scary man, a very imposing, warlike king. Impressive to his subjects, certainly - but a loving, affectionate dad? Maybe not so much. So I think much of Hamlet's hesitation comes out of a fear to acknowledge his true feelings about his dad. He wants to be all Fortinbras, mobilizing armies in his quest for revenge, but he ends up more Laertes, saying the words and claiming that he'll do whatever must be done, but then never going through with it. (It's always been my position that what causes Hamlet to finally act is vengeance over the death of his mother.)