Hamlet "thinking too much" certainly supports my theory of Hamlet's tragic flaw being inaction! There are many examples of times when Hamlet over-thinks a situation. For instance, early on in the play, Hamlet is brooding over his mother's new marriage to his uncle (claiming it incestuous) often right in front of their faces:
A little more than kin, and less than kind! . . . O God, a beast that wants discourse of reason / Would have mourned longer--married with my uncle, / My father's brother, but no more like my father / Than I to Hercules. Within a month, / Ere yet the salt of most unrighteous tears / Had left the flushing in her galled eyes, / She married. (1.2.65,150-156)
Also early on in the play, Hamlet, realizing that Claudius is trying to find out why Hamlet is acting strange, decides to "put an antic disposition on" to confuse his uncle (1.5.172). This certainly exacerbates the situation and, some say, leads to Hamlet's true insanity.
Next, there is the famous scene in the church where Claudius is praying and Hamlet over-thinks his revenge so much that he doesn't go through with it:
Now might I do it pat, now 'a is a-praying, / And now I'll do't. And so 'a goes to heaven, / And so am I revenged. That would be scanned. / A villain kills my father, and for that / I, his sole son, do this same villain send to heaven. . . . No. (3.3.73-78,87)
Finally, Hamlet's famous monologue (where he basically contemplates suicide and rationalizes why he is unable to avenge his father's death) is probably the most famous instance of over-thinking this world has ever known:
To be, or not to be: that is the question: / Whether 'tis nobler in the mind to suffer / The slings and arrows of outrageous fortune, / Or to take arms against a sea of troubles, / And by opposing end them. To die, to sleep--/ No more-- . . . Thus conscience does make cowards of us all. (3.1.56-61,83)
Of course, all of these things indirectly cause the tragic ending of Hamlet. If Hamlet hadn't obsessed over his mother's new marriage, the play would have ended before it began. If Hamlet hadn't pretended to act crazy, he wouldn't have become crazy in reality. If Hamlet had killed Claudius in church, the finale would have been in Act 3. If Hamlet had killed himself when he was contemplating suicide, there would have been no Hamlet to participate in the rest of the play. Each instance has the same result: no final fencing match, . . . and no bloodbath. But then again, what fun would that be?!?
I know it's been answered, but I make mine shorter and simple.
Hamlet's inability to act. He plans and plans, but in the end does not act upon them. Not on the plans he had formed anyway.