> The GPL is only restrictive in that it does not allow one to take
> code into a proprietary state.

I actually have no strong opinion on whether a software license should
try to restrict proprietary changes or not, but I dislike the GPL
because of a small clause which no one talks about much (and which a
lot of people ignore, including us):

       a. You must cause the modified files to carry prominent notices
          stating that you changed the files and the date of any change.

I find this pretty restrictive.  Anybody but the copyright holder of
the software has to maintain a prominent version history inside the
file.  Which is a real pain if you're also maintaining a version
history in CVS or whatnot.