Summary:
I believe the spectacular performance of W.R. Berkley (WRB) can mainly be explained by four reasons: an owner-oriented management which is more or less run like a family business, smart capital allocation, disciplined underwriting and a long-term focus. The Berkley family has proved themselves as very good insurance operators.
W.R. Berkley has proved to be an excellent performer, but gets almost no coverage and followers. I assume one of the reasons is its low dividend yield: A dividend has been paid for 43 years, but the yield is rather low at 0.7%.