These are very thorough answers, but in short the world did not take Hitler seriously because he was not their problem. He was targeting Jews, an unpopular people, and he was in a country far away targeting other countries we were not allied with. Until he attacked England and France, we stayed out of it.
One reason not mentioned yet is that Hitler had the enthusiastic backing of many of the Germany people as well as a highly disciplined and authoritarian power structure. People in the democracies were reluctant to go to war (as the behavior of Chamberlain at Munich suggests), whereas Hitler spent his first six years of power grooming the German people, psychologically, for possible warfare. Early victories fed support for the war and contributed to the confidence and enthusiasm of German soldiers.
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The early German success in WWII was due to the fact that Germany was much more ready to go to war and had been planning for a war for years. By contrast, the Allies had simply been hoping that no war would occur.
For years, the German military had been planning ways to reverse their loss in WWI. This led them to devise new tactical doctrines such as the doctrine that came to be known as "Blitzkrieg." In the mean time, the Allies were doing no such thing because they were not driven to avenge any loss. Instead, they were simply hoping that the sanctions would prevent German remilitarization and that outdated tactics like the use of the Maginot Line would be effective.
Since Germany had new tactics and was ready for war, it was able to win many early victories while the Allies "played catch-up."
One way in which Hitler was able to be so victorious (for as long as it lasted) was his exploitation of the suffering economy of the 1930s in Germany. As seen with the French Revolution, it is easy to change the face of a country when poverty and discontent rest in the hearts of a country's citizens. Hitler proposed that the "expansion" of Germany's borders was necessary to Germany's economic improvement so that it could be come a "major power." (Recall that Germany had suffered heavily for its part in World War I.) In this expansion, Hitler insisted that these newly acquired "territories" should receive a serious "housecleaning,"—
...the current population of these territories justifiably ought to be enslaved, migrated, or exterminated, and re-populated by Germanic settlers.
The first areas Hitler had in mind were Poland, the Ukraine and Russia. His perception that the inhabitants of these areas were "inferior" led him to believe that success would be easily achieved. The strategy he supported was to engage in a series of small wars, where "lightning attacks" (blitzkrieg) were used. This process would provide a marching-like movement, quickly from one target to the next.
Hitler also kept potential threats from stopping him, with "empty promises" made to Arthur Neville Chamberlain, Britain's former Prime Minister (1937-1940), and...
...treaties with fascist and imperial cohort nations (Italy, Spain, Japan.)
The speed with which Hitler moved and the echoes of World War I, blinded some of the world's major powers to what they faced in Hitler's aggressive movements. At first, Britain and France (two Allied nations) hesitated in becoming involved in the war with Germany especially because of the recency of WWI; however, watching the fall of Czechoslovakia, the two countries agreed to unite if Poland were ever attacked. Despite their agreement, Hitler attacked Poland, but he failed to recognize the "turning point" of the war when Britain and France stepped in as agreed. Before long, Hitler's movements became more erratic:
Hitler's strategy became more and more based on paranoia, intuition, flawed logic, and unrealistic assumptions...
A great deal of Hitler's success was his ability to quickly "suppress" any movements that challenged his authority. Fear was a major tool that he and his troops used successfully for some time. Hitler's "madness" moved him to engage the Allies "despite the strategic costs and failures." One strategic failure, for example, was the Third Reich's attempt to dominate and/or destroy Britain's Royal Air Force (RAF). In 1940, in the Battle of Britain, the RAF...
...defended the skies over Britain against the German Luftwaffe, helping foil Hitler's plans for an invasion of the United Kingdom...
The RAF dominated the skies during the war—especially in the "strategic bombing campaign" against Germany, which—with help of other Allies—served to slow (and eventually stop) Hitler's progress across Europe and into England itself.
In the final stages of the war, Hitler's military strategies lacked "coherence." It would seem that the element of surprise and the recent "costs" of WWI at first slowed the Allies' ability to successfully respond to Hitler's advancement through Europe.
Additional Source:
http://en.wikipedia.org/wiki/Royal_Air_Force