So I went home after last week's lesson, made some changes, expanded the nonpitched percussion break, and basically decided I was more or less happy with the piece, which sounded like this:
Unknown Precipitation 2/21/10 by Kyle Vanderburg

But then, I somehow managed to get a lesson with David Maslanka, who gave me several great comments on writing for percussion, writing for marimba, and just writing in general. I went home, reworked a few parts, and came up with this:
Unknown Precipitation 2/23/10 by Kyle Vanderburg

And now, one more composition lesson to take it to, and hopefully there will only be minor editorial changes.