Originally Posted by Abomination:
“It's like Midnight all over again. We never found out what was out there, what it's ultimate goal was...and that's part and package (and power) of the story. And yet some people still think we'll get an answer someday.”
To get specific for a moment, I think there's a certain story-telling difference between 'ambiguity' and 'uncertainty'. I suppose it's a little like the difference between 'imply' and 'infer'.
Many stories go along the lines of "Is the villain evil or just misguided? Are the monsters out to get us, or to save us? Did the heroes do the right things by blowing up the base?" Even if you don't get an answer, the story is presenting you with the options to consider.
Listen was like that, up until the end - the question was always, "Are these things the actions of undetectable aliens, or something more rational? Is the Doctor paranoid or perceptive?" The story actively presented us with two alternatives. Of course, in Listen's case, it then collapsed them down to one certainty.
Midnight is subtly different. It presented a situation that had no answer, didn't give you any answers, with the characters having to make decisions based on no information other than preservation of their own lives. There's not much else to form debate around.
I would call a story ambiguous if the writer is implying possibilities. When the inference is in the hands of the viewer, it's more of an uncertainty.