There's no such thing as a bad story. Written work goes hand and hand with "art", which is subjective.
There can bad storytelling, flat, uninterresting characters, lack of conflict, no development, so on.. There are rules to art too.
I personally wouldn't expect a modern (lets count the prequels as modern) Star Wars film to follow all storytelling canons, to have complicated character development an so on.. It's like we should already be familliar with those characters. Sure, Qui-Gon seems very flat if you watch him in the movies, but if you actually go and read about him in the EU, you realize he is quite interresting (even though the EU doesn't exist for Lucas..)