Why is that from NY to Germany is a lot faster or shorter than Germany to NY?

the wind. when flying from NY to germany, wind is also assisting the plane. whereas flying from germany to NY, the plane is flying against the wind so the resistance slows down the plane, thus making the trip a bit longer.

the flight plan. same for from here to NAshville is shorter time but back is longer!

I heard that it would be the other way. Germany to New York would be faster than New York to Germany. I could be wrong though. The reason is most likely correct. The world rotating that gives us night & day is what makes it take longer in one direction than in the other direction.

It's because of the winds following the Coriolis force. That means that in the northern hemisphere winds (and ocean currents) are rotating clockwise (think about the Gulf Stream going up near Iceland) and in the south hemisphere counter-clockwise. That doesn't mean that it also should happen in your bathroom like many people are saying because actually coriolis force only has effects on large masses (like an ocean). It also has nothing to do with the fact that the plane is flying in the same direction of world rotation because that would mean that it's faster going to NY than coming back and also that the plane is "outside the system of relative forces called world" which is not :P Here is an image with oceaic currents: http://upload.wikimedia.org/wikipedia/commons/6/67/Ocean_currents_1943_(borderless)3.png You can see that between NY and Germany the currents are going to East but near the equator they're going back to West. The map is a bit old but it's clear, maybe you can find some other reference.