No one has to log-in to visit your public blog. No one has to log in to submit a comment. It's up to you to set your Discussion Settings up exactly as your prefer. This page > Settings > Discussion is where you set up defaults for comments.
Other comment settings
_Comment author must fill out name and e-mail
_Users must be registered and logged in to comment
Note that the prompt to submit and email address and a website URL will still appear but providing an email address and logging in are not required.
You can post instructions for your visitors in a text widget if you wish. That's easy to do. You copy and paste this into a text widget:
Readers: Note that the email address and logging in prompts do appear for comments on this blog, but providing an email address and logging in are not required.
See here and read about how to set up a text widget. http://en.support.wordpress.com/widgets/text-widget/
P.S. Do be sure to set up full comment moderation ie. "An administrator must always approve the comment" because anonymous commenting is bound to increase the amount of spam your blog will receive.