We ported to OS X in about three weeks, and we have an outstanding pull request for FreeBSD support (https://github.com/rethinkdb/rethinkdb/pull/688), so as long as it's a Unix-like operating system, there a reasonable amount of work involved.
Porting to Windows is another level of complexity altogether.
slava @ rethink here. I would love to get a windows port (if only because it will give us the option to use Visual Studio's debugger). Unfortunately it's a rather non-trivial undertaking. I'd like to get this done some time within a year, but I can't make any promises.
