I, personally, love playing games like R-Type Final, which has 3D graphics and environments, but gameplay is confined to the second dimension (ie. there are corners, but they are automatically traveled on; you do not have to turn manually). Then there are games with 2D graphics and 3D environments, which allow for movement in 3 dimensions thanks to the ability to jump (many action RPGs for the GBA are like this, including M&L:SS). Then there are games with 2D graphics that are confined to 2D (such as Super Mario Bros.). Finally, we have games like those you'd find on a current- or next-gen system, which have 3D graphics and take place in the third dimension.
I hope that helped you.