Login itself is the biggest obstacle. If your blog is not as popular as say HN, then don't bother with requiring logins to post a comment.

Seriously no one wants to bother even creating a user login just for yoursillyblog.com. It's pointless. What about spammers? So what, create a moderated posting queue if that's an issue. Just don't make me sign in. This is your silly blog, not my bank account.