For years, schoolteachers have used formulas to determine the “reading level” of text, using factors such as the required vocabulary for words, grammar, sentence length and structure.
This technique has been proven to be statistically valid for printed material, and it's interesting to see that the technique is now being applied to the web.
Check out this web readability evaluation tool
It does not disclose the decision rules, but in my limited testing it seems to be quite accurate.
In my tests it rated web sites and blogs as “junior high school”, "high school", "college level", "graduate level" and "genius". It liked my web site:
It also seems to have uncanny insights into the value of some blogs: