I think one thing that every writer should strive to avoid is tunnel vision.
It's easy to fall in love with a story idea. In fact, it helps the writing if you love what you're doing. That passion can carry you far and motivate you to stay up many a late night to work on your script. The problem can set in when you become so fixated that you forget the larger context in which your script has to exist.
In other words: at the end of this, someone has to buy the script.
The writing needs to have appeal beyond your narrow, "Well I've always wanted to tell this story." If your passion is 18th century colonial shuffleboard, you might find it hard to attach a backer who's equally passionate. It might be the best damn colonial shuffleboard script ever written, but what does that matter if there's no market for it - or no one who will even read it?
That's an extreme example, though. What about a script where some young male writer uses the premise to explore his unusual sexual fetishes? I've read more of those than I care to know. The same with stories from people treating their own lives as fan-fiction. In these, the lead characters lives are full of misery. Their parents hate them, people are mean to them for no reason, their boss tries to rape them, their landlord robs them blind, their kids hate them - or have a terminal illness - and society seems out to get them. Through it all, any sympathy for this lead is erased by the shrill and obnoxious way they whine about how hard life is and how they need to get through it.
This is usually solved by the character meeting their perfect, too-good-to-be-true mate and then getting a windfall of cash in an unbelievable way. If you're dealing with a real hack, they'll have their character win the lotto and solve all their problems that way.
Sure, you want to write it, but is that really the kind of movie someone wants to see? Is that the sort of movie YOU'D go see if given an option? I've know a few writers who love a particular genre of movie, but then write a completely different kind of film!
It's always great when you can finish a script, but knowing what that script can do for you in the end is also important.
