Basically a lot more can be achieved in a 3D environment, just compare GTA vs GTA 4. 3D worlds are better able to drag the player into the experience.
I love many 2D games, Super Mario World, the fancy Galaga remake Warblade, the fancy Turrican remake Hurrican, classic c64 and Amiga games, etc. But such games are less demanding and are more suitable for online distribution due to small files sizes. The PSN has some good 2D games, Dark Myst looks interesting.







