On the PCE, the hitbox is a virtual box "layered" on top of the main character sprite?
Most games do that. It's called a bounding box.
I use boxes that are slightly thinner than the sprite so you don't get lame deaths like where your pinky finger pokes a fireball and you die.
I liked reading this. It was pretty neat.
The most important part of a platformer is getting the controls right for the jumps. If you f*ck that up, all your smooth scrolling and flawless collision detection ain't shit because it'll be a pain in the dick to move around.