In Shakespeare's Julius Caesar, what are the tragic flaws of the protagonist?

Julius Caesar has at least two tragic flaws as he is depicted in Shakespeare's play. Those flaws are ambition and hubris. There can be no doubt that he aims to become king of the Roman people or that he will want even greater aggrandizement after that first step. He maintains a modest profile up to the point at which he is about to be crowned king by the senate. He knows that his ambition and his hubris would work in his disfavor if the masses knew about them. It isn't until minutes before he is to be crowned that Shakespeare has him reveal his tragic flaws for all the Romans and all of Shakespeare's English audience to see. Brutus and Antony may argue about whether Caesar was truly ambitious, but Shakespeare's audience should have had little doubt about the truth. Here is a flagrant example from Act III, Scene 1.

CAESAR
I could be well moved, if I were as you;
If I could pray to move, prayers would move me;
But I am constant as the northern star,
Of whose true-fix'd and resting quality
There is no fellow in the firmament.
The skies are painted with unnumber'd sparks;
They are all fire and every one doth shine;
But there's but one in all doth hold his place.
So in the world, 'tis furnish'd well with men,
And men are flesh and blood, and apprehensive;
Yet in the number I do know but one
That unassailable holds on his rank,
Unshaked of motion; and that I am he,
Let me a little show it, even in this;
That I was constant Cimber should be banish'd,
And constant do remain to keep him so.

Hubris is defined as excessive pride or self-confidence. Caesar has demonstrated supreme hubris throughout his entire lifetime. He thinks of himself as being different from ordinary men. In Greek tragedy it was common for the gods to punish hubris. The gods did not like being rivaled by humans. This is interesting because the gods were both superior and inferior to humans. They had been created by human imagination and could be overthrown if humans ceased to believe in them. That is what has actually happened to the Greek gods--so they were right in feeling threatened by hubris. 

Shakespeare seems to have created the speech quoted above in order to show his audience that Brutus and Cassius were correct in their assessment of Caesar's character and their forebodings about what Caesar would become if the Senate made him king. But Caesar was such a powerful figure that death could not stop him. His spirit continued to overshadow Roman history for many centuries. His nephew Octavius achieved what Caesar intended to achieve. Octavius became emperor and after his death an immortal god. The successors of Octavius also became emperors and gods and continued to bear the name of Caesar.