Hello there Solo, and welcome to the Guild! Hmm, let's see here. One thing I noticed right off the bat (though unrelated to your initial concern) was that it looks like the HSV Noise some how spread beyond the cloud shapes on the "Trees Copy" layer to the rest of the layer as well, hence the background is all grainy looking. Not sure why this is, unless GIMP interacts differently on Mac 0SX then it does on Windows XP. It's an easy fix though. If you add then apply the "Trees Outline" layer mask to this layer it should get rid of the Noise. OK, on to your question:

  1. Hard Edges: actually, this has always been an issue with this tut. I have not yet been able to figure out a way to isolate, and then bump map portions of a clouds layer without it ending up with hard edges. Which is why I opted to try and "cover up" the edges of the forests with individual trees (see the update on making individual trees).
  2. Fluffiness: I know that you said you followed the instructions exactly, but the clouds in the image you posted do look different, and I'm not altogether sure why this is. Did you by chance remember to select "Turbulent" (Step 1: 2.b.i.) when creating the Solid Noise layer? If you forget to select Turbulent the trees will look allot flatter. However if this is not the issue, let me know and I'll try to go through the tut step by step and see if I can figure out where things might have gone wrong.