Hello? Surely you are not unaware of XEmacs?
If that isn't an incompatible implementation
of a major tool, I don't know what is, and both
implementations came from the same codebase,
which was (and both still are) under the GPL.

People with conflicting interests and ideas
will always be able to create incompatibilities,
regardless of what licensing model is used.
(This is not to say that I agree with Sun's
position, only that open source licensing is
not such a potent magic that it can rid the
world of problems.)