What do people consider to be the weight of a "lightweight" pannier?
Compared to backpacks, panniers have to absorb more impact and friction damage. If your pannier melts at the first crash it is not much good.
I think people should look for weightsaving in the stiffening and mounting and closure systems before sacrificing fabric weight. Many panniers use large plastic mouldings or heavy board.
Zippers or flap closure is one point of debate. A zipper is heavier and more highly stressed than a flap top, esp when overloaded.