While it makes sense that getting stabbed in the belly by a lightsaber would be fatal in some way (there are some rather important organs in there), it doesn't really make sense that Qui-Gon died within minutes of getting gutchecked by Maul. He wasn't exactly bleeding all over the floor and his brain/heart/lungs/other immediately critical organs didn't look like they got hit; even his diaphram must have been fine if he could still talk.
Or maybe Qui-Gon is a species that holds their important guts in their gut?