Scriptwriting is a fantastic way to frustrate the crap out of you. Even if you conquer everything - plot, characters, theme, etc. you still may completely miss the mark - selling it - if you don't pay attention to the tip of the spear:
The Concept. (warning: I'm going to whip this spear analogy to death)
A high concept is (loosely) defined as being "the elevator pitch" - something you can say in a few sentences between floors of an elevator ride. I say parking lot pitch because it's even harder to contain someone who is searching for their car and anxious to get out of wherever they are - elevators are simple. Unless the person you're pitching is an action hero and can escape through the roof, they're stuck. Parking lots not so much - but I digress.
A pitch can also be the logline (I'll give you examples below)
BULL (new series on CBS): Based on Doctor Phil's early life, a psychologist who is a world-renowned jury analyzer solves crimes every week.
BRAIN DEAD: An alien species invades people's brains in Washington and makes them even more partisan than they are causing even more gridlock in the halls of power.
Columbo: A seemingly bumbling detective who is actually a brilliant crime fighter, solves the HOW DONE IT instead of the WHO DONE IT.
The concept here is to pitch someone quickly and concisely so they request your script.