How Long Should a First Draft be?

I have reached the 60% mark on my work in progress, and I've decided to let it marinate for a short time while I do some re-writes on my previous story.  It's the first time I've stepped away from a book that wasn't like 90% done.  It makes me nervous.  I don't want to be a non-finisher.  But I've poured out all the really exciting and fantastic stuff for the current book (now it's just down to that whole tying together and smoothing it all out part) and my mind has been generating new ideas for that other book.  Against my better judgement, I've decided to let the errant brain do it's thing.  Maybe once it's got that other stuff out of it's system we can come back and work on the WIP with renewed vigor.

I wonder though, what is a first draft?  I'm sure the answer is, as it always is with writing, that there is no set rule and it's different for everyone and every genre.  But is there some sort of average?  If I shoot for my final draft to be between 70,000-80,000 words long, what would you call the first draft?  When you're done writing the story would be the obvious answer, but I have a habit of spitting out all the thrilling stuff first, you know- action, magic, sex- then going back and giving it more substance (which is always in my mind but hasn't made it to paper yet at that point because, well, I know all that back story stuff already, duh.)  This time I'm pausing at 42,000.  Eek that's short! But is it too short for a first draft?  I like to just have things FINISHED.  I hate having things left undone.  But I suppose being outside my comfort zone is good for me once in a while, right?  RIGHT?

The first draft is just you telling yourself the story.

-Terry Pratchett

Check out Karen Miller's piece The Tyranny of the First Draft