Which brings me to this. World-Building is important people, particularly in Fantasy and Dystopian genres. Nothing will kill credibility faster than a poorly made world. Keeping with The Selection for the moment, we have a world filled with castes (nothing wrong there - caste systems have leaked through every society on Earth, whether rigidly enforced by religion or entrenched through money) but beyond a mention of some poverty issues, very little was explained. Then we have the rebel groups. Beyond being called Northern and Southerners, and that one bunch were killers, that was all I knew about them.
When writing credible fantasy/dystopia/futurist, it's important to create a world that seem plausible. Melanie Rawn, a fairly prolific fantasy author, created a trilogy called 'Exiles'. This is a matriarchal, fantasy set on a different planet far in the future run by Catholic Separatists... And that's the biggest problem with it. Leaving aside the magic chuckers, the basic premise of this matriarchal world is 'Women run stuff cos they get pregnant', which makes no freaking sense, particularly as women get pregnant everyday in 'our' universe, and weirdly enough it hasn't given us the power. Plausibility is a bonus, now if Melanie Rawn had determined that only women had a particular power, or that it had always existed thus, without the need for ancient space traveller nonsense, the book would have been easier to swallow.
There is a proviso to the above. If you can write convincingly well, or craft some supremely wonderful characters, then world building isn't as important. But this is only going to work with certain authors. So make your world building work.
If you're writing dystopia, and you can't work out why America would turn into the hellhole you've decided it becomes...don't try to write it. Do a Suzanne Collins and set it so far in the future, and muddy the history so much, that no one can say it wouldn't happen.
If you're writing fantasy and want a world of centuries long seasons, or countries of only one season, actually think about how that would impact people. Because at some point, some smart-alec will argue that 'x can't happen because what are these people eating'
Know the rules and history of our world, if you're writing sci-fi. Realise that things change, but more things remain the same. Human nature has altered little in the past 2000 years, but cultural values have. It's no good taking Western society and giving it a hard-core patriarchy or overly heavy government without thinking about how that would happen.
If you think about the above, your world building will improve, and as a consequence your book will be more likely to succeed.