Oddly enough...it was the storytelling. Odd, right?
My brothers and I had the awesome good luck of booting Ninja Gaiden II in the NES way back in the day as our first ever game. It was vague, and in english, but we could sort of make out some plot progression that had to do with a genuine motivation to set shit right in the world, rescue your girl, find out what happened with Jaquio, 'The Demon', and it had everything; from crazy levels, even crazier bosses, to surprising plot twists: That Jaquio guy from the first game (that we didn't have, back then) was a genuine tough ************ and...he wasn't DEAD...not truly...and he was the one pulling ALL the strings and NOT Asthar, whom he actually used, and the latter dude never even found out.
I was an impressionable kid, and right then I thought "This is way better than the movies or TV." And I was damn right.