I'm not even sure I was playing on a regular DS, or if Tecmo has created some kind of new dual-screened system that is appropriate for action games. It was just right. It certainly wasn't natural at first to play something like Ninja Gaiden with the stylus, but understanding set in very quickly. What felt at first like randomly stabbing at my screen quickly became strategic. It's actually significantly easier to target enemies directly (by drawing on them) than it is to juggle multiple enemies with a joystick.
Gallery: Ninja Gaiden Dragon Sword
The game starts you off as Ryu's apprentice (or something) Momiji, a female ninja clad in white. She walks through an expertly-rendered hilly 3D landscape populated by, you know, a couple of monsters, some bats, the basic ninja lifestyle stuff. As an introduction to the gameplay, first you learn how to move, then slash monsters, then block, then throw shuriken at this awfully inconsiderate person who shoots arrows at you. The top screen, of course, features a map, which is fine -- the less non-fighting stuff to worry about in an action game (like where you are or where to go) the better.
The sort-of-iconic vertical wall jumps are handled by repeatedly slashing upward. The iconicity remains, but the annoying, frequent falls back down are gone. That's pretty welcome.
I should also mention the graphics. It's still a DS game, so the 3D's not perfect, but the slight jagginess of the characters is ameliorated by extremely smooth motion and lush 2D backgrounds.