1. Not giving enough information for your reader to be able to picture the world in their own minds. As with flash fiction, it is the telling detail that matters here. Your story should ensure it has everything in it the reader will need to know.
2. Not making the world attractive enough for the reader to care what happens to it. The details you give them have to matter. The readers need to see what is appealing about World X always doing this or that on their equivalent of a Tuesday, to name one example. They also need to see why it matters. In this case, would the world collapse if the tradition changed?
3. Not having the right balance of character types. You do need a decent villain. You also need a hero/heroine with the right "can do it" attitude. Yet if you just have the good guys, there is literally nothing for the good guys to fight.