The discussion was a comparison against the security culture in Windows. Source availability is a sort of bare minimum at least, obviously by itself thats not enough one wants the source to have been crosschecked by lots of eyes.

My personal opinion regarding security I would say is easy ability to apply sandboxing at various levels on the user side for software we trust less. (By user side I mean not depending on the developer to package the app a certain way etc. A virtual machine or a chroot jail etc are examples of what I meant by this).