The secret to a good opening is it must entice you into reading the rest of the story. It can do this by giving a fantastic sentence or two that means you must read on (George Orwell's 1984 - "It was a birght, cold day in April and the clocks were striking thirteen" is a wonderful example of this.). Another great way is to start with dialogue that again is strong enough to make you want to find out how the conversation ended and what the results were from it. Or a character is introduced or referred to but there is something intriguing about them shown right from the outset, again making you want to read on and find out more.
So I think there is definitely room for improvement on once upon a time!