A few reactions to this:

1) Jennifer's Body failed because it wasn't any good. Tone wasn't the key problem. Or if it was, you can attribute a lot of it to number 2...

2) Good characters with strong motivations give you a lot of leeway with tone, atmosphere, premise, whatever. Zombieland worked because it was a classic coming of age story with classic, strong characters whose motivations drive the plot in a sensible, relatable way. We can accept the whole world of Zombieland more easily, because we totally accept the way the characters behave within it. Columbus wants to get home, wants love. The girls and mentor want to protect themselves and find a bit of happiness in a ruined world, but find goodness they respond to in Columbus. Jennifer's Body was just a bunch of stuff happening and characters saying clever things. Another example of one that works is Kiss Kiss Bang Bang. Sure it's stylized and hilarious, but all that style works in part because all through the convoluted plot, everyone has REALLY strong motivations.

3) A lot of tone is the director's job. You need to give them something to work with (or to want to direct in the first place), but as a script reader, I read a lot of "tone" that was really just trying to paper over a weak script with "feel."

4) Don't think about tone. Just write it. If you've developed yourself as a writer through lots and lots of practice, what your feeling/thinking/attempting will likely come through unconsciously. Then you can find out from trusted readers if it worked.

Just my two cents.

Jennifer's Body, while no great shakes, was actually a lot better on the page. It needed a few rewrites to really sing, but those never happened. However (and this goes to Amos's third point), Karyn Kasuma butchered the direction in numerous ways -- and one of them was getting the tone all wrong.

That being said, while I agree with most of Amos's points as to the success of Zombieland, I think a major factor was that it was primarily a comedy. Can you think of any genuinely scary moments in it? Even the gory parts are mostly played for laughs. To me, it's a horror comedy in the way that Ghostbusters is a horror comedy -- the genre stuff is just used as a basis for jokes. This style tends to be a lot more appealing to a mass audience than a true horror/comedy, that mixes scary scenes with funny ones, and is usually is destined to be a cult success at best.

One of my favorite movies doesn't get much press. How come?

http://www.youtube.com/watch?v=avShicv_Y90

In my Copious Spare Time (©), I teach on the Creative Writing MA at Edinburgh Napier University in Scotland. My colleague Sam Kelly offers this definition of tone, which chimes pretty well with you, Scott:

TONE = the narrator's attitude to the events of the story.

Good examples of handling tone in horror comedies? I like Shaun of the Dead, and An American Werewolf in London.

Those two are pretty much the only two true horror-comedies that were mainstream successes, so I would agree (though even Shaun was more of a cult thing in the US).

I think the original Scream strikes a very strong balance, at least for the first two acts, as does James Gunn's Slither and the upcoming Attack The Block.

Oh, and an underrated British flick called Severance.

Examples of horror-comedy done poorly... too many to name, but off the top of my head: Black Sheep, Dead Snow, and Rare Exports.