Who are you writing for when you start telling a tale? This is something every author needs to consider before starting a new book. "Myself," you might think, but if you're only writing for yourself you'll likely never publish professionally. Another bit of advice I was reminded of while at Dave's workshop is audience analysis.
I've started more books than I care to admit. It's a bit embarrassing to tell someone I've started X number of novels, but finished 0 of them. (Clue to aspiring writers: finish your damn stories!) Part of the problem I run into is missing my audience. I got 15,000 words into a fantasy YA recently--was having a fun time creating it--when my writing group unequivocally told me it wasn't YA. I was very upset with this. I was doing everything right. My protagonist was fifteen, all the adults around him were powerless while he was the one acting. There was an explosion of action by the end of chapter one. I had a (I felt) strong voice for my character. The story was very much YA. "No, no," they said. "Your protagonist thinks like an adult. No modern teenager will connect with this guy." Blah, Blah, Blah.
I still feel their reaction was a little off, since none of them read the type of fantasy I read. However, after fuming for two weeks I realized that a lot of what they said had merit. I wouldn't change the story at all to fit their taste, being confidant in my own style and story creation, but this character and story probably aren't fitted well for YA. I was writing for an older audience.
So make sure you know who you're writing for. The big western fantasy I've been working on for a year and a half isn't going to appeal to most younger readers. It's absolutely meant for adult fantasy readers. This means I can include themes and content suitable for my audience. It means I can tell the story I want to tell. YA is still an area I'd like to write for, but I need to educate myself better on those types of books.