Hi Taska, thanks a lot for the detailed feedback!
So, about the controls, specifically regarding the momentum issue, I think it's mostly a matter of getting used to... I had quite a few other people play the game and normally this is not a problem for them. I understand it is not perfect but it is incredibly hard to calibrate the controls to feel comfortable for everyone :P
On the other hand, about the slopes I really see it is a problem, but I have improved it in the next version that I will upload soon.
Regarding the use of the items, I think you're right and I will probably change these as you suggested (the key thing was already suggested by someone else as well).
Finally, regarding the collision problem in level 1-2, thanks for finding and reporting it, I will be sure to fix it for next version too. :)