Ah, worldbuilding... The difference between the best dystopians and the ones that should still be in the slush pile.
Worldbuilding can easily make or break a book for some people. I, personally, am not one of those people, though I can see why it bothers people. It is mostly a concern in dystopian and fantasy novels, both of which I love to read. A world that isn't fully realized by an author is a world that's difficult for a reader to imagine. We're left with more questions than answers and are so consumed by the questions that we can't see past them to the story and the characters.
Now, I may have an uncommon opinion on worldbuilding. Like I said, it's not going to ruin a book for me. It's not even really a concern when I'm reading. Seldom do I even think about worldbuilding when it's not brought up by someone else. Generally, dystopians seem to be narrated in first-person and the narrators are by no means experts on whatever science brought their world to its state or even the details of it. In first-person narration, the reader knows what the narrator knows. It doesn't bother me in the least to not know quite what's going on. As the protagonist learns about the world they live in, so does the reader. In the third-person, I do expect more details, but I still don't expect much unless you can tell the narrator is omniscient.
For example, in Wither by Lauren DeStefano, a large complaint from readers was the worldbuilding. They didn't find it believable because the economic, political, and/or scientific situations weren't viable to them. But the book is in first-person, narrated by Rhine, a sixteen year old who has been living in fear and a little wildly with her brother. So, like I was saying, Rhine isn't going to really know a whole lot of what's really going on in the first book. She's been protected for her entire life and knows only what she's been told. I don't find a huge problem with the world being a bit of a mystery for that very reason. Sure, I had other problems with the book, but worldbuilding was not a problem to me.
Another example of this is Defiance by C.J. Redwine, which also suffered from readers claims of a lack of worldbuilding. This is one example of when worldbuilding was subpar, even to me. It was immensely difficult to determine if the book was fantasy or post-apocalyptic. Basic, basic stuff, but it took quite a while to figure it out. Even I have standards that have to be met.
In general, I think there's a general amount of information about a world that should be presented to the reader so they can form an idea in their mind. At the same time, I, personally, don't need every small detail about a world, or even an explanation as to why it became like it is. As long as it's remotely plausible, the characters and story are engaging, and the writing is decent, I can be sold on a book. :)
What do you think of worldbuilding? Can a lack of it ruin a book for you? Why? Or maybe you're like me, is it mostly an afterthought?