I've lost track of the #Gimp devs I was talking with on Masto earlier... but two things about the rotate tool:
1. Don't change the transparency mode while rotating. I've got two scans of a large sheet of paper and I'm trying to align them -- so I have the top layer in "darken only" mode, so I can see how they align -- but as soon as I start to rotate, the back layer disappears.
2. PSP had a tool where you draw a line over a piece that should be horizontal, & it gets corrected. Gimp lacks this.