There are only two major platforms that realy matter nowadays.. and that's Windows and Linux.
Every major Unix platform now has Linux compatability as a high priority and most Linux
developers consider cross-platform important, so it works out that OS X, FreeBSD, Solaris, and
friends can be essentially treated as 'Linux' with a little effort. Most of these sorts of
differences are abstracted away from common app developers by the GNU stuff.
The temptation is that by supporting Windows as a primary OS open source projects can
dramaticly increase their profile. They can attract media attention, get financing, and
hopefully attract developers. These are all good things.
The bad things about supporting, as I see it, Windows is that your going to massively increase
your support costs and make a number of very-easy-to-do things in Linux very hard because now
you have to do it the Windows way also.
There is one major project that I know of that has a Windows port, more or less, but refuses
to release it. This is Ardour, a significant audio production application for Linux. They
don't release it because they feel the costs of supporting Windows users with common audio
problems is to much for them to handle. (something like that, ardour folks forgive me if I am
wrong) If somebody that is a expert in Windows and is willing to spend all their time
supporting Ardour on it then they'd probably release it.
Other effects of Windows on Linux stuff I see is things like in popular high-level languages
they tend to shun forking and using sockets for IPC and concentrate on making dozens of
different high-level schemes for doing shared memory IPC and multi-threading stuff. Most of
which has litle significant advantage over traditional 'posix-style' methods in a Linux-only
world, but matter dramaticly in the Windows one. (At least that was my impression.)