How do I know if an idea is good enough to write?

I got this question the other day from a member of my writing group. "How do you guys determine which of your ideas to put in the drawer and which to devote time to? "

This is an interesting question. As I look back over what I've done I kind of have a process:

Usually I put every idea into the drawer. If I keep pulling it out then I start to consider it. Basically If the idea is cool enough to keep me interested it might be good enough to keep someone else interested. This is true of every story that I've finished, and especially my larger works. Here are a few signs that It might be interesting:

• I start making notes about the idea on scraps of paper 
• I imagine a short scene or a line of dialogue or a particular action sequence 
• I can imagine an ending or a direction for a story 
• It just gets under my skin so much that I want to tell someone else about it (though this is usually a bad idea because the other person can never see the whole picture in my mind, that's why I need a whole story to write it) ** 
• If the story is interesting enough to work on I start to judge the length. I don't know how other people do it, but I tend to have a good sense of how long I can write about something. Is there enough there to fill a whole movie? Is it just a short story? A novel? And epic? If you stop and think about it, you might be able to guess how much a single idea can last. 
• With a few scenes or conversations jotted down I then try to figure out a basic plot outline. If it's a short story this is usually in my head, if it's something longer then I write it down. 

Usually if the idea gets to the writing phase at all I push it all the way through. However, I currently have 3 and maybe 1 more project that I started seriously and haven't finished. Only one of those projects was started with a clear goal, and that goal was about 1/3 of the way through the book....incidentally I only finished 1/3 of that book.

In total I have 20 projects in my "This is cool enough to write about" box. Those are all ideas worth writing about. About 13 of those are full length novels, and two of those are series. This doesn't even include my scripts, board games, or other projects.

Another one of my group brought up the idea of many irons in the stove. I love that saying, but right now I can only concentrate on one iron at a time. I imagine that as I move forward into a career as a writer I will get back to working on multiple projects at a time, but right now I'm really focusing my energy on getting my current project right.

So to wrap this all up, I can tell if an idea is worth keeping if I can't get rid of it. I can tell if it's worth writing about if I write about it and I want to write more. I can tell if it is solid enough to write a book on if I can plot out a whole book (or a short story if that is your goal). If I balk at any one of those stages then I don't write. Ideas are easy, finding an idea that can carry you through an entire project is gold.

What are your ideas? How do you decide whether an idea is worth writing about?

** Writing out an idea is important, and no one really cares until the whole thing is done. I have a few ideas on the topic here. I heard recently that a writer begins a story, but only a reader can end it. Basically it's important to present a completed project if you want someone to see what you are thinking. Otherwise you're just using them as a brainstorming partner which is different than engaging them in a discussion about a complete idea.