Different camera movement methods in a 2d- side-scrolling game that makes it fun

Question

Possible reference to this question, but I want specific answers pertaining to Side-scrollers.

IMO, side-scrollers can be classified as

1. Player doesn't move, the background graphics move to fake player movement.
2. Player moves also background moves, but background movement is dependent on the (x,y) movement of the player > I find this more appealing
3. Player can't move, the background moves at a fast speed. Depending upon the obstacles coming along with the background the player has to perform actions like jumping, ducking, charging, etc...

I would like to know what all other things can be added to the gameplay so that the above mentioned 3 side-scroller categories can be made more addicting.Please mention the category of the side-scroller in your reply.

Edit Modified the question title to reflect what I actually want

Answer 1

I've written pretty extensively about implementing "dead zones" in the camera for a side scroller (or top-down game). The dead zone is the area in the center (or sometimes off-center) of the screen where moving your character does not cause the camera to move. This can reduce motion sickness and almost always makes the game feel a little less amateurish than if you completely lock the camera onto the X-axis position of the player. One of the subtle things that can make a side scroller more playable is the value you choose for your left-side or right-side camera buffers!

For example, I made a short video demonstrating the (very narrow) dead zone in Super Mario Bros 3: http://www.youtube.com/watch?v=89TRXUm8jMI

If you want to learn the basics of implementing a dead zone, check out the article I wrote that I linked above. For an in-depth explanation of what different kinds of dead zones can make the player feel, check out Steve Swink's book Game Feel.

Answer 2

(If you talk about shoot'em'up)

I'll be old fashioned, I'll be not innovative but you can't do a shoot'em'up without a very well done and rewarding power up system and without epic end level bosses.