When I first started outlining my current WIP, I created character analysis for my main characters, a plot outline, and also some basic information about all the cultures I might mention in my text. (All cultures in my text are fictional, so I wanted to line up the basics so they didn't run together.) Lots of nice back story, which I enjoyed.
Now that I've started writing the first draft, I've noticed that a lot of the back story isn't making it into the text. All that mythology I made up is skimmed over, and other thing are getting a vague mention, if at all. Is this bad? Is there an apparent lack of important world building going on here? Am I skimping on details because they're apparent to me but might not be for the uninitiated?
In my view, I think I've got it covered. I think a lot of the back story I came up with doesn't actually need to make an appearance in the book. It was nice for me to know how these people might feel about an event, in case I decided that someone wanted their input, but a lot of the details were just for me. Something to remember as I flushed out personalities that no one else needs to care about. The world building was useful for me as a writer, but I don't think it was anything the reader is going to miss.
Maybe a future beta reader will come to me and say, "I don't understand why these people insist on doing this thing," and then I'll return to my back story and put more of it into the text for the reader. Then, it will be nice to know I've the information on hand. Until that time though, I think it's better for the information to remain in my notebook and not in the novel.
For other useful information on Need to Know, check out this post by Lady Glamis.