"I'd say tone is really the feel of the piece - it's what sets expectations really. The tone of the script helps you to understand what to expect from it - and so when something leaps out that feels unexpected (and not in a good way), it's often because it's tonally not a fit.
(An example that springs to mind for some reason... is when I found myself watching Deathproof at a festival in Amsterdam... I sat down in the midnight hours, a little worse for wear, thrilled to be treated to an outdoor rom-com watching experience with all my pals... I saw Kurt Russell (and was hoping for Goldie Hawn) in a rednecky sort of bar, flirting with a barmaid who I think may have been wearing his cowboy hat. He offered to give her a ride home... So far, so good. She even satin the back of his car... All very chivalrous... He asks her if shewants to go left or right... She makes the mistake of giving the wronganswer and before you know it, her brains are dripping down the window.
As the colour drained from my face and I reached for a sick bag, I realised I had made the error of misjudging the tone of this film! All the signals were there for rom-com fun and suddenly, I was inhorror-ville. (Yes, I'd got the genre and style wrong too - but it feltrom-com like in tone if you ask me (for those few minutes anyway)... And then suddenly blood and guts a go-go).
However, if I'd been aware of the title, my expectations would have been set... and had I not been so inebriated, I probably would have questioned why a field full of blokes were so eager to watch it! Guess the point I'm trying to make is how important tone is in setting expectations - and what can happen when you misjudge it!
Establishing the tone of something is crucial really in defining what it is and how it should be executed - before every drama begins shooting, there's always a big tonal meeting where the director (informed by the writer of course!) sets out his vision and all the HoDs talk about how they're going to achieve this... (Sure you know all this but just thought it might help in your exploration of what the flip 'tone' really means).
Tone is the defining characteristic of a piece really - you can have two very different dramas about the same subject that are differentiated by their tone... Bad example but Waterloo Road vs. Teachers... Same territory, very different shows, very different attitudes. In fact, attitude is probably another useful way to think about tone - you can have two cop shows (same genre, same subject) but their attitudes can differ hugely and that, I'd say, was down to tone."