What if Hitler doesn't formally declare war on the US shortly after the Japanese attack on Pearl Harbor? I have no doubt that the US would eventually have joined in the fight against Germany, just like the Soviets joined in against Japan later on - but with Germany and Japan not being official allies, the US and Germany weren't technically at war after Pearl Harbor, and it wasn't the US that changed this state of affairs. What would have changed, though, if Germany hadn't declared war at that point? When would the US have declared war (assuming a more or less similar course of WWII), and what would have been different until then?
(One thing I can think of is that the amusing joke about the Hungarian diplomat delivering the declaration of war to the US wouldn't exist, but I'm looking for more major effects.)