I'm amazed, here on my 4th draft, how much the plot is still evolving. It's almost comical to look back on my first draft now, and the outline that led to it.
To you aspiring writers out there, slogging through that first draft and feeling down on your story, all I can say is... plow forward. If your outline is as weak as mine was (in hindsight), it's still good enough to hammer out a first draft. I'm convinced now that only after doing that can you see where the flaws are, and where the diamonds lay.
Revising is hard work, and not nearly as fun as the first draft, but I think it's where the real magic happens.