I think Zahl has it all wrong. It's not the lack of real names that creates the ethical problems. Sure, it might be nice to be able to know who to sue (though I wouldn't be adverse to appearing as 'Friskiness T. Gunrunning' if it came to that). But the bigger problems have more to do with the form than they have to do with obvious anonymity.
It's all Rush Limbaugh's fault.
No, seriously. Blogs have a lot in common with Rush's old "rant and take calls" format than they do with online fora or newsgroups. It was his program that established the "dittohead" phenomenon, where Rush postificated and his groupies called in to give "what he said" responses. Blogs are prone to the same sort of behavior, except amplified in the usual way the computers allow.
Successful blogs develop a following, and that following is rather often of like-minded people. And especially if the blogger comes on strong in expressing his positions, it's pretty likely that contrary responses get jumped on by the crowd. The result can be a kind of gang attack. But surprisingly the ethical problems appear when the blogger tries to rein this in, since his forms of control all involve some sort of censorship. Some people are just disruptive, and there's not much that can be done about them except ban them. But almost anyone with a contrary position can be taken as disruptive, because the most harmonious state is where never a constrasting view is heard. And that's where the really pernicious problems lie, because the temptation will be to falsify the history of the discussion by deleting or altering posts. And in religious discussion, there is the further temptation of interpreting contrary views as being immoral not just in their content, but simply in their stating. The expression of dissent is then transformed into bad manners-- and therefore it can be suppressed.
Thus tendency is to turn a blog into a little pool of the like-minded, openly hostile to dissent. And the world of bloggers turns into a bunch of little armed villages.
Bad manners? I don't think ending anonymity is really enough of a protection against that. After all, Dr. Zahl himself has been pilloried at length for "uncharitable" and excessive acts and statements he has made in public under his own name, as for instance when he refers to "the steamroller of what we now call 'revisionism'". It was an absurd statement, but putting his name to it didn't seem to curb his tongue. And conversely, as computers amplify anything else they offer the opportunity for the anonymous to abuse an e-mail address by bombarding it with abuse too.