When Stack Overflow (SO) was new I did a lot of Java work and I quickly noticed that the best answers were from on particular guy and I remembered his name - Jon Skeet. For some time I thought he was a Java expert and only later learned that he is the author of "C# in Depth" and his specialty is really C#. It still amuses me today that even as a C# expert he probably knows more about the intricacies of Java than many Java experts.
That being said I think that Jon's exceptionally high SO reputation is not only because he is really good but also because he has earned a lot of it very fast and very early on. Unlike other platforms (HN for example) your reputation on SO still increases over time if you stop engaging or only engage lightly with the community. Given the rules stay the same as they are today I don't think it is possible for any new user to ever surpass any of the high rep early SO users like Jon, VonC, Hans Passant or Marc Gravell. I don't think the same is true for HN but I'm not entirely sure.
That being said I think that Jon's exceptionally high SO reputation is not only because he is really good but also because he has earned a lot of it very fast and very early on. Unlike other platforms (HN for example) your reputation on SO still increases over time if you stop engaging or only engage lightly with the community. Given the rules stay the same as they are today I don't think it is possible for any new user to ever surpass any of the high rep early SO users like Jon, VonC, Hans Passant or Marc Gravell. I don't think the same is true for HN but I'm not entirely sure.