how much detail is too much?

This probably varies from writer to writer. Personally, I'd say you should use as much detail as you need to communicate your story to the reader. You don't want to jam in details that don't actually matter to the story, but you also want the readers to have a clear idea of what is going on.

Some writers like to leave a lot to the imagination of the readers, but I'm kinda vain and tend to want the readers to imagine the same thing I imagined when I wrote the story, so I'll often describe things in quite a bit of detail.

 

Don't leave anything out that is important to your story.

A lot of world-building is needed for fantasy, but all that work is just the backbone to your story. Description is wonderful if it creates a rich world for the reader, but overkill if it becomes the focus.

 

The student has become the teacher!

Well done, grasshopper

 

The point has already been made about making your story worthwhile enough to warrant a meticulous level of detail, and to not bludgeon people with too many facts. The other half of your question centered around leaving things unexplained. Leaving parts of your story open to interpretation has the potential to be volatile, and must be handled with care. We've all read stories in which plot elements were left unresolved to "heighten the mystery," but the result was just irking us readers. The mystery has to be sufficiently enticing and well-crafted in order for you to sell leaving it up in the air. There isn't a guide or list of helpful pointers for that sort of element; it all comes down to the craft of the writer. It also comes down to trust. Readers are willing to travel down some strange roads with a writer who's established a sincere bond with their storytelling.
Basically, what I'm trying to say is don't leave things unanswered unless you really believe it'll make sense for the story, and you won't just irritate readers.

 

If a leave any mystery in anything I write it is with the intent to explain it later on, usually for the sake of the plot. Or just as importantly to have the reader learn about the setting as they read, the growth of the world so to speak.
I think this brings up the question though of what exactly is meaningful detail to the story. If I go on to describe the details of an article of clothing and it's history is this meaningful to the story? I argue that it is very important because it describes the small minutia of the society that created this article the same way the cut of a dress tells a lot about the culture in which it was created. I could tell the reader, much more quickly and concisely but it would leave out the immersion into the setting.
But I have noticed I do not follow the normal way of thinking, I love details, to me they are what actually creates the story. The plot is important but the details are what makes it real. I say put as much detail as you can. I'm not saying describe every microsecond of every moment, but I want to see and feel the brushstrokes of the work you create! That's just my opinion though and I know others think differently.

 

I wouldn't advise you go too into detail. I remember one story I read and for two and a half pages it described a King's throne room. It just went on and on. The reader doesn't have to know every single detail, just give basic details to give them a good idea. Always remember that no matter how many details you give a reader, they will always imagine it differently than you. That bieng said, don't overwhelm them with details.

Let's take an example of a character's appearance for example. I describe a character I have created in my story. From my description, you will be able to form a mental picture in your mind of how he/she would look. This is an amazing thing that our brains can do, but we don't need to know every detail about them. I say, say what is most important and then let the reader fill in any gaps with their own imagination. Lot's of times you can tell an author just went overboard and was jsut trying to use up more words. Good luck.

 

It varies with what the main purpose of the story you are writing. If it is more about the world in which the story takes place like very much how LOTR is, then being more descriptive is the way you want to go. If you want more of the event or action for the plot of your story, then you have to be more vague because the purpose is getting your characters place to place, event to event. Here are a couple of books that helped me decide how much detail should go into my novel.

-Characters and Viewpoint by Scott Orson Card
-How to write fantasy and science fiction also by Scott Orson Card

It helped me a lot and I hope it helps you too.

-Good luck

~Tera-lon~