Good game, I beat it across two days while I was eating lunch at work.
I love the single button concept, reminds me of "the Impossible game" on the xbox/iphone...
It's true that this game is very trial by error, but not much you can do about this without pissing more people off.
On normal this game is easy, getting the medals makes it a bit more of a challenge... hard mode is not too much more of a challenge. The difficult part is wrapping your mind around the single direction.
Fix stage 20 on hardmode or include a hint, no matter what I did the game just slows down when i reach the door and then i get killed by the spike... even tried timing it just right to maximize the time i was at the door and that didn't do anything. Read through some of the reviews but no luck.
one thing you might want to consider is making some ridiculously hard way to move backwards, like double left click-hold so that the people who can't break out of their mindset that you have to be able to move in all four directions will shut up.