We've been watching Heroes with the kids. I've never seen beyond season 1, and we're almost at the end of the first season now, so all I know of the show is limited.
However, as we were watching the last episode we saw together, Patrick asked a question.
"If Peter can regenerate how come he has a scar?"
As it turns out, this had been the subject of online debate and discussion long before our viewing with the kids, but I'm not really concerned with the answer at this point. What stood out in my mind is the simple truth that if you don't maintain your own internal consistency, or explain a significant change, the audience disengages from the story. It loses some of the intrigue and adoration of fans when they're pulled out of the story and legitimately think the writer has made a mistake.
I think, if you've been reading everyone's posts on The Dark Knight Rises, some might argue that the final installment of the trilogy lost the plot. I didn't see it that way, but it took seeing the movie a second time to be able to fully explain why. There was one critical detail near the end I hadn't really processed the first time around, but it made the end make sense to me. I take no conclusion as absolute - not that Bruce wouldn't return, not that "Robin" wasn't about the become the next Batman - and so I'm not as bothered by the ending as some have been. In fact, I liked it, and the audience in two packed theaters both times I saw the movie cheered.
They were certainly satisfied.
That said, I think it's often easier to have the audience overlook character consistency and some plot holes in movies, and to a lesser extent, on TV. In books, readers are far more likely to skim back and double-check.
My advice to writers is simple. Don't underestimate your audience - we have a ten-year-old and eleven-year-old critiquing book and TV stories in this house, and no, just because they're kids doesn't mean they'll read anything and think it's okay. If they think it's crap, they'll tell us why. That's how a huge fan of The Hunger Games ended up giving up on the third book in the series. It didn't hold up.
The other tip I have is to know what the core of your story is. If you do, it makes it harder to lose the plot... If an 11-year-old can notice when it doesn't hold together and make sense just imagine what an editor or agent will say if your story doesn't stand up against its own established internal truths.