To start off, the game's controls are very stiff. It feels like you have to really mash the left and right keys in order to move. The way you can have the sword out and enemies will walk right into it isn't as satisfying as having to slash at them individually.
The graphics are trying to be something like Atari but the Atari 2600 could do just a little better than this, you could have drawn the ninja a pair of legs and a body and it would still look atari-esque (but the limbs would have to be made up of squares and rectangles)
And finally, when you die the screen freezes, there's no option to restart the game other than to refresh.