I can't make a direct comparison, but Paul Preston claims that the Spanish Civil War has created nearly as much volume of English language scholarship as has the Second World War. That is a staggering amount of writing. Sadly, I do not have the numbers that Preston used to make this claim so it is difficult to measure the two on that basis. Given how involved much of the world was in Spain--volunteers from all over the world, major involvement from Germany, Italy, and the Soviet Union, pointed "non-intervention" from France, the UK, and the US, the religious dimension, the atrocities, the art, the literature, the history, and more--it is not surprising that there has been so much interest in the subject. So if I were to point to a civil war that has captured the imagination on the same level as the US Civil War I would nominate the Spanish Civil War as a strong contender.

I'd like to first question the basic concept: is there any evidence that the US Civil War is actually 'uniquely regarded' outside the US at all? I would've thought outside the US this would be a fairly obscure piece of history, maybe only relevant because it meant the end of slavery in the US.

To any people, their own civil war will probably take presence. England's Civil War led to the rise of Constitutional Monarchy and everyone here knows far more about both that and the War of the Roses (an earlier, Dynastic conflict which was essentially a civil war).

The Japanese will put more emphasis on the Boshin War, the French on the Revolution, the Russians on their Civil War, as with the Spanish and Japanese. Many developing nations have civil wars that have only just ended.

So honestly, no. We've all heard of the American one because America is the World's sole superpower, the War wasn't special in comparison go other Civil Wars but as all media is so American Centric, everyone is familiar with your history. And to be clear, by 1861 America was far from Small, it was continent spanning and one of the Great Powers. America hasn't been an underdog since at least the 1830s.

Is it because we were one of the last nations to abolish slavery and that made the war a grotesque world set piece?

We weren't one of the last nations, not even close.

Was it because some generals, like Sherman, became famous beyond our borders?

I doubt many outside the US heard of Sherman.

Or is it possibly just because the U.S. in the 20th century dominated the movie industry and our pop culture clout simply spread knowledge like that?

Bingo.

I don't think you are too far off. Reading old Vietnamese newspapers and journals from the 1940s and 1950s, the US Civil War is mentioned quite a bit. But so too is the Chinese Civil War that was ongoing at the time. These writers are preoccupied with the topic because Vietnam was immersed in its own civil war from 1945-1955 and they drew on notable comparisons.

The Chinese war is mentioned because it was contemporary and had a great impact on Vietnam's own fate. The reason why the US Civil War is mentioned has more to do with the course of American history. Usually the authors wrote about their hope to avoid a hugely destructive war like the US Civil War. But, they note that the US was able to persevere after the war and still defeat the Axis countries and become one of the world powers after WWII. Their hope was that Vietnam could do the same.

The ACW was rather unique insofar as you had a collection of semiautonomous states secede from a larger union of semiautonomous states in order to form a new union in the belief that they had a legal right to do so, but were subsequently brought back into the previous union status quo pre bellum.

Switzerland, afaik, is the only other country to have ever experienced a similar civil war scenario.

I think it has to do with the sheer scale of the civil war. It mgiht also have to do with the amount of new technologies that were put into use. The telegraph, the railroads, ironclads. All were used on a scale never before used in warfare.

Now, I might be wrong - I'm more interested in Latin American history, but I think this might have hade a lot to do with it being so well known.

I think it is easy to feel this way, given that the US has an outsized influence on the English-speaking parts of the internet.

I would bet my second-born child that a Chinese-language version of /r/AskHistorians would not have ACW questions every day.