Well, specifically for gaming, you *are* already rendering the scene once, all 3D requires is another render. Adding support for it is fairly trivial.
Something that's also important is making sure that
everything you render has some sort of depth information associated with it, so that the video drivers can render it in 3d. That's where most games that don't work in 3d fall down. The HUD is at a mad depth so you see it double, that nice reflection on the blade of your sword is flat on the screen while the sword's beyond the screen, the surface of water is an upright sheet facing you, not a flat surface stretching into the distance...
Plus, if you have whole screen pixel shaders they're not going to work in 3d. Lot's of blur, HDR lighting or fog effects could muck up, depending on how they're implemented.
Support for 3d is perfectly possible, but the right decisions have to be made at the beginning, and some effects might be much faster to render and quicker to implement if they're done in a 3d unfriendly way.