Yes, MO's data will be available in a form which would make porting to a F/OSS app possible, but isn't right now. Beta 7 of SE will be rolled out Monday or Tuesday, and will include a way for me to get an XML dump of the database. Right now, there isn't an open alternative that I think is worth switching to, but there are some in the works.

(I wonder if MO's data is available in some form which would make porting it to a F/OSS app possible, were it to exist?)

var renderForm = function(postId, jDiv) {
      var jForm = $("#form-comments-" + postId);
      if (jForm.length > 0) {
          var form = '<table><tr><td><textarea name="comment" cols="68" rows="3" maxlength="' + maxCommentLength;
          form += '" onblur="comments.updateTextCounter(this)" ';

Changing the rows="3" in the line starting with var form=.... into rows="8" might also do the trick, slightly more cleanly.

The small text area for comments has always bothered me too (though I use Chrome, which allows easy resizing). I've used css to change the height of the box to 130px, which should be tall enough to fit 600 characters.