Which idea?

How do you decide which idea you're going to spend months (possibly years) writing?  Novels take a lot of time and effort, well they do for me, given the day job, need for sleep, soul-saving time w/friends, occasional trips to see family or go to Europe, and so on.  And there are so many books in my head that could be written, half-formed premises, ghostly characters, rip-snorting scenes, even the occasional musical number.

I won't know for sure that I'll write the book based on an idea untill I outline it and make sure it works.  But what makes me get to the outlining stage?

1. A protagonist I want to spend time with.  She must be likeable but flawed, with a sense of humor and a sense of purpose.  She's strong, usually a leader of some kind. 

I did this exercise in a writing workshop I took where you do this:
      a. Write down three people (real or fictional) who are heroes to you.
      b.  Now write down three qualities in each person that attracts you to them.
      c.  Those nine qualities are issues you will confront in your life and/or qualities you possess or will work
           on possessing.

Me, I wrote down:
Nelson Mandela - wisdom, leadership, integrity
Robin Hood - compassion for less fortunate, derring-do, willing to risk life to fight injustice
Queen Elizabeth Tudor - ability to overcome misogyny, leadership, cleverness.
(I started off with Cleopatra in this slot but scratched her out - then realized she has the same three qualities as Elizabeth Tudor, so it works out the same.  Plus Cleopatra didn't have to be the "Virgin" Queen.  Bonus points for that!)

And it turns out I like to read about these types of characters and to write about them too.  I get impatient with characters that are slow, humorless, sheep-like, and entitled.  I can bear it if they're clumsy or not amazingly wise as long as they have some of the other qualities in spades. 

2. A plot that provides enough conflict to sustain a novel.  Conflict keeps people turning the pages.  It's the essence of a story.  There have to be big obstacles standing in the way of my protagonist's desires, and the stakes must be big.  I don't have to know the ending or the middle, but I get a feeling in my gut that there's lots of juicy stuff to work with here.

3. An idea or hook that's semi-commercial or better yet, really commercial.  I'm not a writer of literary novels.  I read and write commercial stuff.  I get bored with anything else - sorry!  The writing still has to be wonderful, of course.  But no dreary plot-less disquisitions on the meaninglessness of life for me, thanks.  If I spend ages writing a book, I'd like at least a chance at getting it published some day.

4. A theme or topic that digs just a little deeper.  Okay, so I want it to be commercial and a fun read.  But I also want my story to have some sort of meaning or deal with an issue that could be meaningful to someone other than myself.  It's not enough to have it be "true love conquers all" or something like that.  There must be an issue that folks struggle with that isn't easy.  It may not be front and center, but it lurks in the background.

5. A tone or atmosphere.  Getting this straight in my head can be the clincher.  If I get the tone down pat in my brain, then I feel like I just might know how to write this.  Is it melancholy?  (Not my stuff, usually.)  Creepy? (possible...) Action-packed with a sly sense of humor? (Hopefully!)

If it was a TV series, how would it be shot?  Bright and sunny, with lots of colors?  Or chiaroscuro?  Glamorous?  Or dusty and a bit faded around the edges?  I need to sort of see it in my head.  Then I'm ready to roll.

How do you decide which idea to write?