My pleasure. Ninja Spirit captures the essence of ninja in a way that other retro 2D platform ninja games didn't. As you pointed out, the gameplay with its gravity defying jumps gave us the ninja feel, but I think the game's atmosphere is a large part of why this game is so amazing all these years later. If you took the hero from Ninja Spirit and placed him in a modern/generic setting, the game would have suffered greatly. From the run-down temples, to the bamboo forests, to the lightning streaked mountain tops, filled with enemy ninjas, staff wielding monks, and samurai ghosts, Ninja Spirit took you into the mythical world of the ninja like no other. F*cking love that game, man.
I do have two other awesome ninja game recommendation- both modern- that capture something of the "feel" and the atmosphere of the shinobi:
Muramasa: The Demon Blade (Wii) and
Ninja Senki (downloadable game for Windows). Here is a video for Ninja Senki:
If you check 'em out, let me know what you think?