It was the arrival of action greatness.
I remember there was a huge video of some of the first long uninterrupted gameplay of it way back in the TeamXbox forum days that someone uploaded. Even before Youtube. It was the only copy of it, and back then I already had a super-fast internet connection and while everyone was trying to download it, it got taken down for good, but not before I got it completed. So apparently I was the only one that got it. BlimBlim's site was also on the rise for Xbox videos back then so we arranged for him to give me a login to upload it to his FTP server.
I think somehow to this day, no one else had that video till it was uploaded.
Another fond thing about Ninja Gaiden that's not tied directly to playing it is that I still have the Famitsu magazine and Japanese demo that I imported from either Lik-Sang before it got shut down by Sony or Play-Asia, can't remember. It was the first and only way for the public to play the game, and I just had to have it asap.
Ninja Gaiden still blows my mind how it just came out so amazing like that. Like, there was no competition, no real reason for it to have to be so amazing, it just was. Itagaki and OG Team Ninja were just insane to put it together like that.
It might be a small part of why I loved their games so much and why they were so good, but their attitude of "we'll release it when it's done" is what I wish more publishers would take note of, so that their games wouldn't just be good possibilities with half-assed polish.