- The writer doesn't rewrite. (See this article on the Rewrite Process) Most first timers think their first draft is "good enough."
- The writer doesn't listen to notes. Most first time writers aren't willing to listen to honest criticism. They just want to be told that their script is great. This is why most professional screenwriters refuse to read first time scribes. (See: Josh Olson's No I will Not Read Your Fucking Script)
- The writer hopes his/her first screenplay will be a winner. It takes thousands of hours and years of hard work to learn the craft of screenwriting. Everyone's first screenplay...well, sucks. Have you heard a story of someone's first screenplay, one they dashed off in "21 days," selling for millions? Sure you have. Hollywood is full of stories.
- The CHARACTERS are weak. More than any other specific flaw, first time screenplays have weak, underdeveloped, ill-motivated characters. Protagonists are passive. Antagonists are cliches. Supporting characters have no complexity. It is difficult to see what any of the characters' objectives are or why we should care about them.
- The stories lack TENSION. The first rule of screenwriting/filmmaking is "keep them in their seats." This means keeping the audience's attention, through the emotions of hope and fear, focused on what will happen in the future. Most first time screenplays get muddled and lose tension; and then the reader tosses the script aside, not caring how it will turn out in the end.
- Too much focus on "idea" and not "execution." Most first time writers think they have a "brilliant idea for a movie" and take a half-assed shot at writing the screenplay. If there is one consistent lesson to be learned on Quora, whether about startups or screenwriting, it is that "your idea means nothing." This isn't exactly true, but it's mostly true. (See: Does Your Idea Mean Nothing?)
Your Screenplay Sucks!: 100 Ways to Make It Great by William M. Akers
How Not to Write a Screenplay: 101 Common Mistakes Most Screenwriters Make by Denny Martin Flinn
Lastly, if you are hellbent on selling a spec, read Ken Miyamoto's advice, "Write To Sell, People!"