Mastodon gives journalists the freedom to run their own servers outside the influence of a centralized authority which may be beneficial for journalists who are reporting on things that could land them in legal hot water by oppressive regimes.

Basically it's if email and reddit had a baby. Get handles of people and content you like and it will appear in your feed.

The nice thing about it is it uses activity pub which means you'll be able to view content from other platforms and sites that decide to use it from a single portal and not just from maston servers. Right now it should be possible to view content from WordPress sites as posts you can comment and like.

It entirely depends on the servers that you join. Some will have more liberal policies allowing any content from any server to come in within reason, others will be walled gardens.

Mastodon is designed to be software first and so how you experience it will be dependent on the configuration by the owner of the server or on how you configure it, if you setup your own just as how email is.