unregistered user can write in blog page – possible? how to?

This plugin allows posts from the front end: http://wordpress.org/extend/plugins/user-submitted-posts/

I have tested it and it worked fine, however, it is basic in that you can get the post title, content, category, tags using this, but if you want to add custom fields or anything more complex you may find a hand made solution works better such as this: http://voodoopress.com/how-to-post-from-your-front-end-with-no-plugin/

Good luck!

Just re-read your question and it sounds like another option might be to use the comments functionality as you’ve suggested but you could create a custom page template where the comments were the main focus of the page, that would probably be an easier option than having users submit their comments as posts.