Gandalf13:
I think we have indeed reached a point of agreement by saying that the Persian army's surrender after the flight of Darius made it easier for Alexander. As with the other points, I don't entirely agree, but I think those are points not as appropriate to be discussed here.
Gandalf13&Parmenion:
I agree with Gandalf that Alexander didn't introduce the siege and phalanx tactics. Siege tactics are more ancient, as already pointed out, documented for Assyria, and even archaic Greece. The Trojan horse, for example, is thought to be a poet's interpretation of a large-sized battering ram.
As for the battle tactics, I think it is important to mention that the Macedonian-used tactic of the aggressive phalanx was neither invented or introduced by Alexander or Philipp II., but by Epameinodas of Thebes in the battle of Leuktra in 371 BC. It is also very simple to understand why: Before, Greek armies were phalanxed hoplite formations fighting in blocks, against each other, both sides usually being equally strong, with the same formation, thus mostly reaching stalemate battles. The victory was decided when one force requested to bury their dead, thus admitting the defeat. At Leuktra, the Thebans faced a Spartan army using this tactic. There was a center and two wings, right and left. Epameinodas restructurized this by strengthening one wing, and making it move differently, encircling the opposite wing of the spartans, thus pressing one side very hard. It was an entire success.
Epameinodas used infantry units for this. The Macedonians used cavalry, which proved to be more effective, as the Macedonian cavalry proved to be able to route the Athenian and Theban hoplites, and later the Persian defense systems.
Parmenion:
Your statement that the Celts of Gaul were backward Barbarians is simply untrue, and follows the simple black-white system of the powers of Rome and Greece. I will not go so far to claim this is rascistic, because it isn't really. It's a prejudice, nothing more.
In ancient Europe, there were three major cultural civilizations: Greece, Rome, and the Celts. Believe it or not, remnants of a well-structurized economical and social system have been found from Austria to Spain and from northern Italy to Britain which wasn't Roman, but Celtic. It was a different civlization, yes, with druids and bloody deities, but this doesen't mean it was primitive. The Romans liked to put it this way out of propagandical reasons, and unfortunately, this mind simply has succeeded until our times.
The title "the Great" has very little to say. There were outstanding leaders throughout history who haven't received this title, like Hammurabi, Vespasianus, Philipp II. (of Macedonia and of Spain)... others however were granted this title for lesser achievements, like Antiochos III., Constantinus, Reza Shah Pahlavi I. ... this, because those people who gave them these titles needed someone to measure with, to look up to.
Finally, a word about the great and glorious Hellen(ist)ic culture: If all the negative, and/or four our mind barely understandable things were carried together in comparrison to the great achievements, quite a lot of people would reconsider their thoughts and ask themselves wether this culture was really that great.