1. There are no new ideas.
The exhaustive lists for movies like Princess Bride and Pirates of the Caribbean, show just how many conventions so many movies share (Even Casablanca and The 400 Blows) . There doesn't seem to be any film or work of note that doesn't have an enormous cross-referenced list of "tropes".
2. The author only writes half the story.
The rest of what makes a successful story live is the audience.
The"Wild Mass Guessing" and "Fridge Brilliance", categories of the site prove how freaking insane fans are, like people who think Sofia from Golden Girls was a figment of Rose's Imagination or that any character who changes actors is a Doctor Who Timelord. With shows and franchises with particularily devoted fanbases like the Recent My Little Pony Series (even just for one character) and the Christopher Nolan Batman movies,the debates just get strange.
But with all this dissection going on over lines or pieces of business that were one-off jokes, or simply trying to re-write plot holes as ultra-subtle brilliance, tvtropes proves that the audience is an active participant, not merely the recipient, of the ritual we call storytelling. 'Cause there's no way in hell Batman's a Timelord.
"The more beautifully you shape your work around one clear idea, the more meanings audiences will discover in your film as they take your idea and follow its implications into every aspect of their lives."