While I have no evidence to support this, I would guess this has to do with the editing process. Comparing this list to http://top.searchyc.com/domains, you see a lot of similar results. The difference is that some of them have quite a lot of results (Techcrunch, Wired), but not nearly as high of a average score.
In fact, it looks like the sites hitting the top of the list for average score are ones that are written by a single author, who has several strong opinions (ZedShaw is a good example). Since Hacker News doesn't allow down voting, it seems that by draw up a commotion, and getting people interested in a semi-controversial idea is what wins votes here.
In fact, it looks like the sites hitting the top of the list for average score are ones that are written by a single author, who has several strong opinions (ZedShaw is a good example). Since Hacker News doesn't allow down voting, it seems that by draw up a commotion, and getting people interested in a semi-controversial idea is what wins votes here.