I don’t know if you have been able to fix your visitor comment problem yet or not, I don’t see any responses.
I had the same dilemma and found out that it was one of my plugins that was the problem. After I deactivated it, my blog now works fine. The plugin I had the problem with is: math Comment Spam Protection. If you are using that one, try deactivating it.
You can also tweak your comment requirements under Settings/Discussion.
Hope this is helpful,
Sue Scaletta