i thought this was pretty insightful

"People complain about endings a lot, but they don't know really what they're talking about. They talk about 'God out of the machine', but they don't actually mean that. What they mean is, you can't win the game with a new piece on the board. You have to have seen already what the downfall of the enemy will be, but not recognized it for what it is. That's what they mean.

Across the board, you have to introduce the element that's going to end your show disguised as something that was self-supporting, there for its own reasons. A self-supporting gag. Done, we don't worry about it anymore, forget about that. Then -- there you go -- it's come back! People love that."

– Steven Moffat (Doctor Who, Coupling)