What one thing do you think caused V to stand out the most in its day and why?
I think the facist themes coupled with the otherworldly nature of the visitors is what ultimately made the saga stand out.
It was a military type resistance/action story with special effects thrown in.

I say V was about the weapons. Come on with the whole nazi motif going on the Visitors weapons were real cool. Plus early on bullets did no good against the Visitors till they got the special bullets. But the Visitors weapons were much more powerful.

In reality probably the Visitor weapons, spaceships, and futuristic technology all added to their superpower, fascist motif which the saga aimed for early on.