All four factors you've listed were important, and the Wehrmacht was a force to be reckoned with. Its only major continental contenders were France, Britain and Russia, the world's largest empires and militaries. It had a medium sized but effective fleet, a large air force, and a massive military with some of the earliest adoption of armoured divisions.

But the weakness of European nations is also a massive factor in the initial successes of Nazi Germany. I will go down in the list of Germany's opponents and show why this helped the German Blitzkrieg and the belief that the Wehrmacht was more capable than it really was.

France: was Germany's main battle to fight before Barbarossa. However, France was so focused on fighting the last war's battles and tactics that their strategy was almost entirely defensive. This firstly allowed German forces to fight on their own time. Whilst the French carried out one short offensive in 1940, no permanent gains were made, and fighting was small scale and even rather civil (a lot of 'live and let live'). Thus the Germans had to make no defensive preparations in mass scale, because France was unwilling to go offensive.

The French were also simply lacking the will to fight. Peace movements were widespread, and many people believed that it was better to be under capitalist fascists than soviet fascists (and France had a very large socialist movement at that time that the rich, military, and industrialists were interested in quashing). The fact that they were fighting for Poland of all places, which of course had ceased to exist, was not a fact lost on the French. Few wanted to lose their cons, brothers, and fathers in a war over people hundreds of miles away.

The phenomenon of the Phoney War had its effect too. After months of almost no action the French soldiers were less prepared and willing to fight, having had time of contemplation and rest.

The absolute lack of any significant defence along the Ardennes (10 of the weakest divisions in the French military) coupled with the fact that the Germans were willing to attack through it with their main thrust, spelt at least massive problems in the French defence, if not a guarantee of defeat. This is hotly contested amongst historians.

Finally, also related to the Ardennes, is the fact that operating so close to the German border the French air force couldn't gain superiority and therefore couldn't bomb the Germans out of the Ardennes (a very real possibility).

All of this combined meant that the massive German military was facing a hugely weakened and unprepared France by May 1940. Although it would certainly be unfair to blame the French defeat on themselves, they certainly hastened their own downfall. France being the main enemy of Germany until 1941 essentially ensured that the Germans could go on unopposed across the continent.

Britain: is the only nation to have fought from the first day to the last, been attacked and counterattacked, and won in the end whilst surviving throughout. However, they certainly had their own weaknesses. A spread-out navy (Far East, Channel, Mediterranean, Atlantic, North Sea, the Indian Ocean, et cetera); a dangerous supply situation in the Middle-East; and no offensive capability on the continent.

The RAF was the most important factor in maintaining the integrity of the British Isles, and even then they only barely just managed to do so. The British situation, until the entry of the USSR and the USA into the war, was dire but not impossible. The most commonly accepted outcome to the war had there been no US/SR intervention is a stalemate peace between Germany and Britain: Britain had no real offensive capability against a conquered and hostile Europe, and Germany couldn't, for the time being, subdue the UK.

Coupled with the weakness of the Far East forces and their total failure against the Japanese (the loss of Malay, the disaster of Singapore (the worst defeat in British history, 110,000 captured and Malay lost), failure to protect Dutch East Indies territory, et cetera) Britain lost a lot of manpower, resources, strategic positioning, and prestige. The Wehrmacht had to deal with less British forces after the loss of the Far East and whilst the Japanese kept more Empire forces busy in Burma. The only advantage to this loss was that Middle East forces were given further priority for supply, support, and armament.

Being an island nation and colonial empire saved the UK, but it also meant that there was a huge stretching of a limited military, and no way to singularly fight back on the continent.

The Low Countries: Simply too small to stand against Germany. No amount of superior training, weapons, and morale would possibly keep the Wehrmacht at bay if you only have a handful of divisions against the weight of the Wehrmacht's main thrust into France.

Poland: Though Poland had a million men in its army, they were using vastly obsolete weaponry and equipment, and faced a war on two fronts with two premier European militaries. Though German casualties were higher than expected (80,000), Poland proved a staunch and brave defender, but ultimately one that was outclassed and outnumbered.

Norway: They had one tank of Swedish design, which was ultimately never used. The Allies could not effectively defend the Norwegians, and the Norwegians had not the strength to defend themselves against many serious military forces (though their terrain aided in defence and resistance post occupation. The German forces with some competent command and fighting skill had little issue taking a lightly defended country.

Greece: The Greeks were, like so many others, simply too small to fight the Germans. Though they were afforded the help of Empire New Zealand and Australian divisions, plus the aid of the Royal Navy and Air Force, and their own forces were unexpectedly good at dealing with the Italian and German forces in a difficult terrain, they were eventually overrun and overwhelmed as the Brits retreated their forces in a hasty evacuation to Crete. Which, too, fell, as a mix of Allied mistakes and German skill.

Yugoslavia: Unfortunately I can't say much in the way for Yugoslavia for I have not studied much of it, but I do know that it was a swift conquering which was hastened by internal politics.

Russia: was the proof that much of the German victory lay in exploiting the weakness of the enemy. Russia was, in 1941, in the midst of military reforms which would have fixed the issues that were faced in the Winter War with Finland in 1939, such ass reinstatement of officers and issuing of new equipment. Being caught in the midst of reform is never a good time to be attacked.

Also most important is the policy of appeasement to Hitler on the end of Stalin. Stalin would not believe that Hitler would try to invade Russia at least for some years. Even when a German soldier with Communist sympathies defected the night before the invasion and warned Soviet officials of the impending attack, Stalin ordered the man executed.

The Soviet air force was simply obsolete in 1941 too, and air power was a massive, crucial part of modern warfare, some would even argue the crux of land-based engagement as well as naval. It would not be until 1943 that the Soviets could possibly have air superiority over the Luftwaffe.

The Hitler syndrome also took place for some time, where Stalin acted as the head strategist for the Red Army. This led to similar issues as the Wehrmacht faced, when being given a commander with little care or knowledge for military issues. Stalin did eventually relegate the military power back to his generals, and men like Chuikov.

A common belief is that the Soviets greatly benefited from the winter. This is to some extent true, however, the winter was in some ways worse for the Red Army than the Wehrmacht. Their tanks faired poorly on many occasions too (the T-26 and T-28, for example, were still common and poor tanks), as did their aircraft, and the severely underequipped infantry also faced problems with cold exposure (though they were not always as accute as the Germans' problems. This point is contested by historians.

Between being unwilling to prepare, not having an intact officer corps (purges of the 1930s gutted the Red Army), interference from Stalin, and moving the vast military industry east of the Urals (to protect from German bombing, though massively slowing production for years), and having a vast but obsolete air force, Russia suffered very heavily against the relatively well supplied and led Wehrmacht.

Summary: Europe was in something of a mess. The only single thing that came close to a joint effort against the Nazis until America entered the war was the tenuous relationship between Britain and France. Otherwise, the nations of Europe were too small, too alone, or too unprepared and in some cases even unwilling to fight. The Nazis thus steamrolled most opposition.

However, when faced against the Western Allies, and the recovering Soviet Union, Germany became very much an equal opponent. The Battle of Kursk is proof: when faced with real opposition, the Germans were just as susceptible to defeat as any other military. Their total superiority didn't exist when fighting opponents that could actually match them and stand up straight. Germany was a force to be reckoned with, of course, but it was not the invincible empire that the media would have you believe it was.

Sources:

The Second World War, by Antony Beevor

The Third Reich At War, by Richard J. Evans

Barbarossa, by Alan Clark

Stalingrad, by Antony Beevor

Crete, by Antony Beevor

Kursk, by Lloyd Clark

El Alamein, by Bryn Hammond

BBC 1973 Documentary, The World at War - Episode 3: France Falls

BBC 1973 Documentary, The World at War - Episode 6: Banzai! Japan

Apologies that this wasn't more comprehensive for each nation, but I feel that it serves as an overview >