I agree that it should be simple at the beginning, but for things like 2D vs. 3D, to be able to switch between them, no matter how modular and separated you are, is going to take a lot of work.
Also, slight side note:
If you would like to get the Crystal Space code from CVS, Astronouth has written a very nice bat file for Win32. Download that
here, from my server. Extract this into any directory and double click on the "crystalspace.bat." And new directory will be created under it. Crystal Space is large, be warned!