Light enough you can move it quickly (ymmv), heavy enough your opponent will have a hard time parrying it with a saber or other relatively light blade. It can be wielded one-handed if you wish to keep your other hand free for balance (important!), shield wielding, or anything else you need to do. It can be wielded with two hands for additional speed (especially important against a lighter weapon), armor-penetration (if necessary at all), or firmness of parry - one can absorb the shock of a parry much more easily with both hands on the hilt.
Wielding a bastard sword, it feels massive yet elegant. Everyone should try it some time. Well, maybe not everyone.