You know I really liked Downton Abbey. Until I didn't. In fact, I didn't even bother to watch the final episode this year, although Ken still wants to and keeps asking (a little plaintively) when we're gonna finish up. (ANSWER: Probably never.) (But don't tell him. I hate it when I'm a hope-crusher.)
I've thought about why I fell out of love with the series and have come up with this answer: I was no longer willing to suspend my disbelief. When Lord Grantham made a lunge at the maid (why?! and why her?!) and Matthew leapt out of his wheelchair because he started tingling and so forth, I just went, "Okay, I'm done. I am not buying any of this any more."
It's a reminder of how important it is to keep your characters believable--they have to act like they would act and not because something might advance the plotline, you know?
What was the moment for you? The moment when you put a book down or turned off a TV series because you weren't willing to disbelieve any more?