Yeah, I definitely agree on all points.
I kinda ran out of ideas that I could easily see a good way to design a puzzle element (the only one I didn't try to pursue was just a variant of the crate, except it didn't rotate, and I couldn't figure out what kind of object that would be). Visual effects as a result of hitting the ghost is a good idea, I hadn't thought of it (then again, the explody ones explode and the crates freeze, and anything that dies has their ghost fade out, so it hadn't really occurred to me--related, the blue barrel's audio clip has a tone-shift).