Well, the concert of Europe wasn't established because of Napoleon per. say; rather it was created on the ideas of the French Revolution which were spread via Napoleon. A major focus (and there are numerous foci which could be discussed forever but lets stick with Napoleon) was that the ideas the French Revolution were way to radical to be left to its own devices which is why numerous nations (from Portugal to the Ottoman Empire) saw this as a huge threat to their monarchies. The idea that the power of kings and sovereigns was no longer guaranteed to an extent (unless power was obscenely abused) was frightening and especially for countries like Austria who were among the first to respond to French, depended on the status quo the most. Austria for the practical reason of if the current system is undermined many minorities will join in on the revolutionary wave and tear the country apart. So multi-ethnic empires and nations with strong monarchies were threatened by this and also the very helter-skelter way the revolution took form, culminating in the Jacobins and the Reign of Terror.

You do raise an interesting point of international law in that there was very poor consistency in the diplomatic scene of Europe at the time. Wars were being fought endlessly (albeit very minor wars of little consequence like (The War of Jenkin's Ear)[http://en.wikipedia.org/wiki/War_of_Jenkins%27_Ear]) and Napoleon forced the nations of Europe to come together to hammer out a lasting peace because of the sheer damage it caused with 3-6.5 million dead in under 15 years (the last time a war that big happened was almost 150 years ago) so to prevent it from happening again the major powers of Europe (UK, Russia, Austria, Prussia, which were the countries that did the real heavy lifting in the war) would always try to keep each other in check and prevent one nation from getting too big. Because France is such a large part of Europe (holding an estimated 30 mil out of 150 mil people in Europe) it was given a seat in negotiations as well. So in effect the Concert of Europe was the closest thing the nations could at the time get to a UN security council. Albeit this analogy is very loose the core elements are the same: it was constant no matter what your political relations, it was used to prevent a major war between great powers from happening needlessly, and preserve the status quo. And it did keep a massive war on that scale from happening until World War I, it failed to address other issues the Great Powers had.