It's easier to define OpenBSD, beyond the obvious stuff (security). It's a system that does not really want to deviate too much from old-school Unix, and a community that firmly rejects any fad or fashion and embraces simple, and indeed sometimes simplistic solutions. That doesn't make it the best system for everything, but at least we know where it stands: the best old-school Unix OS. It's almost as if it lives in a parallel universe where time has stopped in 1990, and the OpenBSD developers are asymptotically converging to the perfect Unix system of that era. I'm not saying that as a criticism at all.
And NetBSD and Dragonfly BSD are also, more or less explicitly, research operating systems where individual developers can come and try stuff.
FreeBSD is still a very impressive project but I think they can do better than being just-like-Linux-but-five-years-later.