Commenting is an important part of blogging since it lets the blogger and readers get to know each other, or talk about relevent information on a post like a forum. However, most people who post on blogs are not registered members since many people are too lazy to join or can’t find the link to join your blog. Comments also make your blog more active, so search engines might visit your site more since it has fresh comment content.
Enabling guest commenting is not a good idea if your blog gets a lot of hateful or spammy comments from guests. Akismetis a good plugin for stopping spam from showing up on your blog, but you have to register for a Akismet key at http://wordpress.com
WordPress by default does not let unregistered guests comment, but you can change this.
Tutorial on turning on guest commenting on your WordPress Blog.
1. Click on Settings then discussions on Left side bar in an admin. account.
2. Uncheck user must be registered and logged in to comment.
3. Modify your moderation settings to allow comments to appear without your approval or with your approval.
4.Click Save Changes Button