With the recent surge in comment spam and the renewed enthusiasm to arrest spam in comments, I have also been noticing a nasty trend in comment verification mechanisms. It might be my own irritability (due to a bout of influenza) that is propounding the problem, so bear with me if I am off the mark.
I read a lot of blogs and I like to leave comments. Your commenting system, be it because of some goofy spam moderation technique or in a futile attempt to curb incorrect XHTML, should not prevent people from leaving comments altogether. Without mentioning any names or blogs, I wish bloggers would consider their visitors to be more important than a few instances of comment spam or an incorrectly formed XHTML comment. If you feel like you need every commenter to register on your blog (or stand up on one foot and cluck like a chicken) or be able to form correct XHTML before they are worthy of leaving comments, you have already lost me as a visitor.
You might have the most well designed and standards compliant blog in the world, but if people cannot use it without expending a large amount of effort, your design is worthless and I, personally, refuse to visit it.