It's official, I am absolutely, totally hooked on Ninja Gaiden. This game is so great because the level of difficulty is high but not for the typical reasons most NES titles are hard. One thing in new games these days (at least the good ones) that people take for granted is the fact that enemies don't react the exact same way with every interaction. There are slight differnces in the way enemies move even if you're doing the same actions to them. This really keeps you on your toes because you can't just pull some "one step to the left --enemy moves to the perfect spot-- then you stomp him" repetetive movements to defeat a vilain. Ninja Gaiden is not the type of game that allows you to memorize enemies movements and simply learn the pattern to beat the level. Obviously certain enemies have certain weapons and movements, but it's when they use them and how they move that makes it great. In order to defeat the number of enemies that come at you, especially in the later levels, you really have to become skilled at the game. The Knife Guys (that's what I call them) never throw the knife with the same pattern, and for that matter, neither do the Flying Ninjas With Stars (oh how I hate them). This is when you really get rewarded from a game, you know you're good at the game and not just the pattern.
One perfect example at this is the simple change between Pacman and Ms. Pacman. Here's looking at you Pinky, Blinky, Inky, and Clyde.