The Persians obviously had no good answer to the phalanx, that had been seen in earlier wars, but the Greeks had never been united enough to take advantage of this.
So with a united Greece, and given his success in the west. Alexander would have been very successful against Persia, possibly over-runing the entire empire. And you would have no subordinate generals setting up their own dynasties in the conquered territories, assuming Alexander lived long enough, which admittedly isn't a given due to his lifestyle and his "lead from the front" military style.
The Macedonians still would have gotten Egypt, and Persia of course had vast wealth. They would have "liberated" the Ionian cities instead of the Persians just handing them over by treaty. But I'm not sure if going east was a better move. Outside of Ionia, you didn't have an existing network of Greek cities to serve as a basis for consolidating hellenistic rule. I suppose they could have founded lots of colonies, in culturally incongruous territory. Plus Persia was a loose knit hetrogenous empire that would have been a headache to rule. I doubt the Macedonians could just finish the job and turn west, and the empire woldn't have been as long lasting as the OTL Mediterranean based one.
Of course the interesting question is whether any of the western Mediterranean cities would have used the absence of the Macedonians to rise to prominence.