In "The Shallows", Nicholas Carr makes a very good argument that replacing deep reading books, with the necessarily shallower reading online or of hypertext in general, causes changes in our brains which makes deep thinking harder and less effective.

Thinking about "The Shallows" later, I realized that laziness and other avoidance behaviors will also tend to become ingrained in your brain, at the expense of your self-direction/self-discipline behaviors they are replacing.

Another problem with the Web, that wasn't discussed in "The Shallows", is that hypertext channels you to the connections the author chooses to present. Wide and deep reading, such that you make the information presented yours, gives you more background knowledge that helps you find your own connections. It is in the creation of your own links within your own mind that information is turned into knowledge.

Carr actually has two other general theses in the book; that neural plasticity to some degree undercuts the more extreme claims of evolutionary psych, which I have some doubts about and am doing further reading on; and he winds up with a pretty silly argument about the implausibility of AI. Fortunately, his main argument about the problems with using hypertext is totally independent of these two.