Let's start with the fact that over 80% of all so-called comment submitted to WordPress.com every day are spam. Once you have accepted that and the fact that the vast and overwhelming majority of all information on the internet is spam, and you know that the number of legitimate comments all A List and B List bloggers receive are very low when compared to their visitor stats, you can put what follows in context.
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 ads 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.