Considering the millions of lives lost in WWII, it does make some sense that the world would be a very different place if Hitler had never risen to power. For example, maybe Stalin really wanted to invade the USA, but the Soviet army was so severely weakened after the war that he couldn't. In that case, if The War had never happened, Stalin might have been able to make this dream come true (or, more extremely maybe the White Russians could have rejoined and beaten the communists back and re-established the Tsardom, establishing Michael Alexandrovich...). Or maybe nuclear weapons would have been invented in peacetime, and without the horrors of Hiroshima and Nagasaki to terrify the world, some naive world leaders would have started a nuclear war without understanding its true destructive potential.
A common perspective is that without Nazism to vilify it, racism as an official and pursued policy of government would have persisted into the present day; despite the current debate over gay marriage and a large multiracial population, people tend to forget that many American states had laws against homosexuality and interracial marriage, and California actually began sterilizing asylum populations twenty years before the Germans did.
The technological impact cannot be underestimated either; NASA (and thereby all its related spinoffs) relied heavily on Nazi-backed rocket research, including use of human subjects, and much of the medical community benefited from similar agreements. Even the research on tornadoes now used to protect the world was based on studies of a Japanese scientist when he learned that his hometown was spared being the second nuclear bombing site due to the weather. The economic slump of the Great Depression was transformed into a period of enhanced American dominance in the world, Japan was transformed from military superpower to economic superpower, and the last vestiges of European colonialism were wiped out. There's no telling if a world without WWII would have turned out better or worse than the world we live in today. And the simple act of killing Hitler may have widely different effects on history depending on the precise time and manner of his death.
Note that this does not apply to aliens/demons or evil time travelers who are allied with Hitler; time travelers should feel free to stop them. Other than that, Time Travel is as useless for solving problems as Reed Richards, if it doesn't in fact do the opposite and create a spiraling Butterfly Of Doom effect as part of a Fantastic Aesop.
Of course, it doesn't seem to dawn on anyone to, say, kill Gavrilo Princip, the guy who shot Archduke Ferdinand and started the First World War, instead ? though even that might well have been only a stopgap; the tangled web of Great Power alliances and colonial hostilities that laid the groundwork for the Great War was so delicate that it would almost certainly have been triggered by something in that time period, whether the Archduke had survived or not. (In fact, it very nearly blew up several times before that point, although none of them are well remembered.) It might be much easier and more efficient just to go all the way back to the 19th century and kill Bismarck, the Prussian chancellor who unified Germany, or Kaiser Wilhelm II, who almost died at birth anyway, or kill Metternich and prevent him from orchestrating the European Concert, or kill Napoleon before he can start the wars that led to the Congress of Vienna... but everyone is just so short-sighted they want to kill Hitler. /quote]
Makes most sense