IMHO there are two things to consider:

• agreeing is easier than disagreeing
• disagreeing is a stronger signal of intelligence than agreeing

If you agree, you signal that you are on the same level as the one you review. When people agree with authorities, it raises their status.

If you disagree, you signal that you are on higher level than the one you review. When you disagree with someone in your circle, you challenge them to status fight. But when you disagree with an outsider, you are only collecting free karma.

Agreeing is easier, you simply write: "After reading the article, I have realized that (summary of the article) is true, and it impressed me deeply." (A few responses on the professor's blog are exactly like this.) If you disagree, you must have some contrary opinion: "I disagree with X, because the truth is Y." In some situations, you can use some cached wisdom for Y, so it may be easy... unless then someone challenges you to fight by disagreeing with Y.

My guess is that the professor liked LW. Therefore students understand that agreeing with LW means agreeing with the professor. For most people, safety comes first, so I would expect most people to agree with LW, and only one or two to signal intelligence by disagreeing. (If the professor would be critical of LW, I would expect most students to disagree, and perhaps one to agree. My expectations are a bit assymetrical, because disagreeing with criticism is weaker signal than disagreeing with praise.)