Well, let's think about it.
1. The license and copyright. 2. The legacy. 3. The userland. 4. The overall quality. 5. Not so many prima donna devels (Theo excluded).
Although the userlands have diverged, there's still a great deal of commonality.
1. I'll give you the license but they obviously no longer share much copyright as they have all been largly rewriten since the 386bsd days. If that were not true then they would all be the same.
2. That's like saying nothing differentiates us from monkeys because we share a common ancestor. I call BS. A common origin implies very little of practical value especially considering how much of the code has been rewritten.
3. So first you say they share a user land then you say the userlands have diverged. Which is it?
4. Overall LACK of quality due to not having a critical mass of developers, you mean?
5. I think they're ALL prima donna devs otherwise why are there so many forks? It's because everybody in the BSD camp wants to take their ball and go home instead of working together on a common goal.
I don't hate BSD or wish anybody ill. These are just my perceptions based on what I've seen.