Basically, it comes down to the degree of power sharing within the top echelons of each respective state. When you think of Nazi Germany and fascist Italy, you almost instinctively think of Hitler and Mussolini. That is because they succeeded in concentrating power to a considerable degree within their own hands.

On the other hand, in cases like Peronist Argentina and 'Nationalist Party' Imperial Japan power was held by a group of elites; in Japan it was the military elite. Argentina is a bit more complex, but can be boiled down to a 'Pretorian Guard' element within the Argentine army.

There is also a difference in their approach to legitimacy. Whereas an autocracy most often derives its legitimacy from the barrel of a gun, fascism derives its legitimacy from chauvinist populism built around nationalistic ideals and built on exclusive provision of welfare to a certain group of people ('nationals').

Basically then, through 'serving' the 'people' (and only the 'people'), fascism accrues power (usually by means of democratic institutions), which it then concentrates in the hands of the head of the party. It is worth noting, though, that 'fascism' isn't really used all that often anymore. Basically, the only states which were actually fascist where Italy and Germany, almost by definition.

Something that makes this analysis tricky is the relative brevity of fascist regimes in world history - and the extent to which the ideology was discredited after World War II.

Communism is a different story: 70 years passed between the Communist Manifesto and the Russian Revolution, matched by another 70 years of the Soviet Union. It means that "communism" can be defined fairly meaningfully, both intellectually and historically, since we have so many writings, debates, and governments to analyze. Of course, there's no single definition of communism because of internal dissension and debate, but it's a framework.

Fascism, as an ideology, doesn't have that. While it arose from various 19th century rightist movements, it didn't really crystalize until World War I. Mussolini came to power shortly after the war, so the ideology of fascism didn't have much time to develop separately from his agenda. The Nazi regime in Germany shared a lot with Italian fascism, but its internal ideology and political theory was unique in a lot of ways. Since it came to dominate the fascist world, it became difficult to articulate what fascism meant outside of Nazi racial theories.

After the Axis defeat in WW2, nobody was too eager to declare themselves loyal to fascism, and later authoritarian dictatorships might have taken elements from Italy and Germany, but didn't work to develop a continuing fascist ideology in the same way.

So as historians, we're kind of limited in our ability to say "this is what fascism means."