I think the swords power came from it's simplisty. It didn't really have any bells or whistles attached, it was a good, honest blade that if it connected directly with anyone, the sheer weight would cleeve them in two without making too much of a racket. Conbinued with his Hidden Mist No Jutsu, it would be hard to sense where he's coming from, and against ninja of a lesser caliber utterly fatal if they didn't know he was coming. Plus it would leave a nice psychological shock to those that discovered the bodies.
The only other blades that are comparible are the twin blades, otherwise every other weapon has a clunky side effect that makes stealth killing impossible (threading needle requires accaccy to kill like sibon and was more of a terrior weapon. Blast Explodes, Blunt is loud, Sharkscale is inefficent at killing quickly and the last one requires intense charka control) hence it is a nice simple weapon, does a good job at what it does, doesn't have any bells on it and it's ability to repair itself makes maintainence less of a pain. Simple, but elegent.