Originally, lightsabers were supposed to be heavy, I believe the specific wording involved the strength needed to move around a huge amount of energy. That's why, in the original movies, you see a lot slower fights, and more two-handed grips. They wanted to show progression of force ability by how quickly/easily you can move a lightsaber around, which is why you rarely see Luke use a one-handed grip (yes yes, I know, the first scene with him holding a lightsaber was with one hand; I wasn't responsible for the continuity editing), but often see Vader use one.
Quite frankly, I'm not sure what to believe anymore, as I don't really like the canon being changed from the original stuff, yet apparently most people think they're supposed to be weightless.
whether you like it or not, a lightsabers blade is weightless, and the difficulty in wielding it comes from the gyroscope effect of the weapon.