It depends on what form of media you're talking about.
In a game, the world is the more important aspect, because it's going to be the world you're playing in, and the biggest part of the game itself. If the world is believable, then other, more mundane flaws can be overlooked more easily.
If you're watching a movie, than the plot itself takes front and center stage; again, because that's the main attraction. What the people are doing is more important than why they're doing it or even who those people are.
Novels, on the other hand, need to have believable characters, because they tend to be the main attraction; while you aren't always seeing the world through their eyes, you tend to see it with them. If they don't come alive, their actions and the world they live in will fail to come alive as well.
That being said, any flaw in any one point, no matter how small, tends to take away from the experience as a whole. Bear that in mind.
'pologies.