Ninja-Saga is one of those games that you can play for hours, simply because there are always more levels to achieve and more creatures to fight. It reminds me a lot of an MMORPG, such as World of Warcraft, but without the MMO prefix. RPGs in general tend to keep me addicted because I love achieving new levels, and I'm inspired to reach the highest possible level in the game.
What keeps you playing Ninja-Saga? Is it the leveling concept, or perhaps the fun gameplay mechanics? Or, to see the other side of the spectrum, what keeps you FROM playing Ninja-Saga? (though if you are here, I doubt that's the case)