It might be of many factors. Persians might not be that good at naval warfare and the sea is a bottle neck separating asia from greece. that and Xerxes might have had a bit of a victory disease.
Regarding Qin unification of the six nations when warfare becomes enlarged in scale and when outcome is decided by not one battle (though it might be decisive) but prolonged campaign - Then it comes down to economics and logistics really. And Qin dynasty took an early step in reform which place it in front of others (reform by Shang Yang)
True. But Persian's Navy are actually quite powerful. The main reason for them to be defeated might boils down to the fact that they are not as experience in that type of terrain and battle. The Greeks on the other hand are very familiar with the surrounding and could make use of these to fight effectively. This is especially true in ancient warfare, it was not just the number of men, the training of your men and your hardware that matters... a large portion of it was depending on your surrounding and how you make use of these to your advantage.
On the Qin unification of the six nations... you could actually credit the Qin's success to one factor - Foreign Talents... why? Many of the ideas are created by talents that are not originated from Qin country. Actually Shang Yang was also not Qin people, but born in Wei.