@Matthew,
"I'm hoping that someone can clear up a point for me. One of the pros with Portal is that since it can sit behind the firewall, Portal is available for everyone in an organization to search and use public items (public in the sense that's it's public for everyone in the organization) without a license, but with ArcGIS Online that feature is not possible....is that correct?"
Your assertion is correct. If you use ArcGIS Online, your items will have to be either shared with "everyone" - it which can there is open access, or if you want to restrict access then an ArcGIS Online named user account is required.
@Adam,
"I can't explain because I am not using it yet, but I would check before I made the assumption that it is able to be used by anyone without a login to search and use "public" items. Is "public a choice within Portal? .. IF Esri allows it, Portal installed behind your firewall could hypothetically be available to users without a login for items made "public"."
Allow me to clarify. If you have Portal for ArcGIS configured in your organization, and you set a Portal item to be shared with "everyone", then yes - people within your organization (e.g., in your intranet network) should be able to access these items without requiring a named user login.
Hope this helps,