Scriptwriting teacher Charles Harris has a very concise way of defining how to write genre scripts (or novels, for that matter):
"Genre is about three things: emotion (the emotion you create in your audience) - expectation (giving the audience the patterns and motifs they expect from that genre) - and surprise (delivering those patterns and motifs in a surprising and fresh way)."
He adds: "One of the challenges of writing in any genre or genres is knowing what emotions and patterns are expected, and finding unusual ways to deliver them."
This is especially true in romantic comedies, for instance, because we know that generally boy gets girl (or girl gets boy, or boy gets boy, or girl gets girl). One exception was "500 Days of Summer," which I liked and did well even though it subverted our expectations. It cost only $7.5 million to make and it took in more than $58 million worldwide, which is a very good ratio.
But it's a risk to not give audiences the ending or even the major beats (plot developments) they expect.
Of course it's also a risk to deliver those beats in a predictable manner...unless the film compensates with the impressive special effects that big audiences for blockbusters appreciate. In that case, the mass audience generally forgives predictability ("Avatar," anyone?).
If you love movies and see a lot of them, probably you're into mixing genres. Conventional wisdom is that this is also a big risk. At the moment, I'm writing a comedy/horror/sci-fi mix novel. Am I asking for trouble? Probably. But I'm having fun writing it...
By the way, it wouldn't be a bad idea when you've finished your first draft to go back over it three times, once each to see whether you have delivered the three elements Harris mentions: emotion, expectation, surprise.
(You'll find many more tips at www.timetowrite.com.)