You’re at a party. The hostess wants to tell you a story, but while the anecdote itself has a stellar punch line, she insists upon giving you a lot of tangential detail as set up. The story itself would have been funny, but by the time it really starts, you’re distracted. Why did she mention the pony if the pony doesn’t play a part?
A beginning is not a beginning when it feels like one for the reader and not the character. Put another way, if there’s going to be a pony, it really needs to matter.
I’ve always felt that a high fantasy writer has a bit more work to do than one writing urban fantasy: we’ve got to weave it all from whole cloth. Both writers have a particular challenge in that they must begin and explain without looking like they’re instructing you.
And I think this is why fantasy queries to agents fail so often: an elevator pitch is damn hard if the world intrudes upon the hook too much. Airships and dragons are familiar concepts. We can face those without too much explanation, but if gravity doesn’t work the same and the sky is pink, we’re going to need a little more to work with. So the world must intrude, and do you see where I’m going with this: balance, balance, balance…
That said, make no mistake, backstory can be the bane of any novel, and the amount of it required to get the story started is directly proportional to the risk of losing the reader’s interest.
The story should start when the hostess’ anecdote is getting good, not during the preamble. That’s when the tale starts for the character.