[mbivert]

As far as I know, the OpenBSD team ensures that the base installation is useful already, so that their "secure by default" claim has some intrinsic value. As a result, even without installing extra packages, you get an usable system, unified (written by the same group of people), well-documented (reading the man pages and knowing what to expect from the software often is enough), easy access to OpenBSD-specific software, etc.

I personally enjoy having not to ask myself questions like, which http server I should be using, and just be rolling with whatever's in the box.

I wouldn't be surprised for Debian, and others, to provide a similar experience, perhaps not as tightly packaged though. I'm not sure the difference is that remarkable either, unless perhaps you have some specific needs that you know are well-managed by *BSD-centered software.