Hardest things to program in a platformer Mario game?

[Mariotroid]

Slopes. Because you have to calculate the positions depending on player's x position and movement direction. Also "y -= 1" is inefficient.

Edit: moving platforms are moderate for me. I managed to figure out some tricks to make it work better. Moving slopes would be a nightmare to program.

Are you talking about angular slopes or rounded slopes? I had some difficulty with angular, but rounded seems impossible for me.