1.Should be flowing at a pace that keeps the audience at the
edge of their seats but still be very comprehensible.

2.If it's a thriller, it should keep the audience guessing, but fooling the audience for too long is not the trick either, keep revealing some, you have to show them Quite early that you indeed are taking them somewhere.

3.Comedy, is always a major component, there should always be a space for that
in your script.A Character, need not be clown to make your audience laugh.
It's the situations that you create using these characters that are funny or hilarious.
but make sure the humor clicks, experiment your jokes before you try them out in your scripts with in different circles. or observe the traits of comedy that are universally appealing ( always keep in mind the breadth of the audience)

4.The slang, plays an important role in shaping the character, and giving him a colloquial touch.Not only the slang or accent gives the character a touch of reality but also at times it appeals to a wide set of audience. (afro-american asian mexican, Texan, italian, indian )

5. Characters, should be identifiable, they don't necessarily have to be doing things that are identifiable, but the layers of their personalities should reveal the humaneness in them.( Even a super-man clicks only if he's shown as a geek )

to be continued...