Secure-by-default, reliability, consistency, and good docs. There's lots of people who would appreciate that even if they weren't technical experts. From there, you have the split of who would be interested in it as-is versus who would use it if someone set up the UI, apps, and so on for them. Mac OS X is probably the ultimate example of layering a great UI for lay people with "just works" mentality over UNIX. I bet something like that on OpenBSD would have significant, even if single percent, market.
