The first is the question of longevity. Web information is not permanent. How many times have you bookmarked a site only to come back at a later date and find the link no longer exists? Essentially this information is gone and can no longer be retrieved.

The second, and more important in terms of information credibility, is the process of peer review. I assume (insert comments here) that when I read a printed article the manuscript was reviewed and edited for technical correctness before being published. This is almost never the case in virtual world.