I spent some time (accidentally) running on the normal difficulty. While I didn't notice anything majorly different, I did notice that in the Oil Rig level on the after taking the first lift, that the UFO comes from the right on Normal, and left on Hard - therein changing the strategy for that part.

I also think there might be more enemies in Hard - so despite my previous investigation into the matter (and what was said otherwise), I've instigated subcategories for the board for the level difficulties and moved runs accordingly.

Due to the majority of runs being on Hard, I've also set that as the default.