I love a well built world. Doesn’t matter if it’s SF or fantasy or historical (all of which require world-building–not necessarily world-creation, but the layering of detail to make a believable place in the mind of the reader). I love the sensation of being somewhere new, picking up the look and feel and smell of a place I’ve never been in my waking life. And I have a pretty good idea of how much planning and outlining and researching it takes. I also have a pretty good idea of what happens when you do too much research.
How much is too much? That’s not as hard to figure as you’d think. Too much research is when you never get to write the book. Too much research is when you hide from writing the book by doing research. Too much research is when you can’t see the forest (that would be the book) for the trees (that would be your thirty-five notebooks comprising the history, religion, fauna, flora, microbiology, climate, reproductive habits, winged ponies, etc. etc. of the setting for the world).
There are other dangers to over-preparation. Having the characters and plot bend and break under the weight of of your prep is a bad thing. But really: the big trap here? Getting so wound up in preparing to build a world that you forget that all that research is there in service of your story, not the point of the story.
Both Rumer Godden (in In This House of Brede) and Dorothy L. Sayers ( in Gaudy Night) have secondary characters who are writing long-researched scholarly books. A small in-joke in both books is that the manuscripts have to be prised from their authors, who want to fix just one thing… This getting caught up in the research thing is not new, nor is it limited to SF or fantasy.
A friend of mine used to have a sign over his desk:
Don’t Forget to Write
Words to live by. I would add: Don’t forget to finish!