We need to distinguish personal annotation from social annotation. Much of Todd’s article seems to refer to the latter (note that this sentence refers to the entire article). Social annotation requires far more than placing anchors.

The case I worked on was integrating public comments into a proposed regulation. As things stand one has a hundred page proposal and a stack of comments. Placing anchors is no problem. The problem is many comments do not relate to a specific place in the proposal text so there is no place to put the anchor. This is not a computer problem. It has to do with the nature of human reasoning.

[comment by David Wojick]