The wonderfully pithy screenwriter John August has a great post up at his blog, johnaugust.com. I encourage you to read the entire thing over there, but the real meat on the bones is quoted below:
“Aspiring screenwriters will often throw a few loglines at me and ask which one they should write. My answer is always, “The one you would pay money to see.”
That’s … scratching your own itch. You’re writing movies you wish existed.
Looking at successful filmmakers — in particular, writer-directors — it’s pretty clear who is doing this. Tarantino makes movies to fill a special shelf at his fantasy video store. Wes Anderson makes movies his own characters would dissect over canapes.
If you have more mainstream taste, great. Embrace that. Scratch your own itch. But forget about “commercial” or “high concept.” If you’re writing a movie you yourself wouldn’t buy a ticket to see, you’re wasting everyone’s time.”