The problem is to write completely compatible code, you have to use ANSI
features rigorously, without exception. The problem w/ this is that the
user has to constantly keep portability in mind when doing literally
EVERYTHING. In addition, various compilers claiming ANSI compatability
actually conform to different versions of the draft!!! This wouldn't be a
problem except that the ANSI committee changed somethings rather
drastically from draft to draft.
All in all, I'll stick to my original statement that porting code from one
machine/compiler to another machine/compiler at this moment is not a
trivial task, as you imply. When the ANSI draft gets approved as a
standard and every compiler conforms to the same thing, and users start
actually writing true ANSI code, this situation may change.