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Here is what a user with 0 trust level can't or can do.
Discourse does not quite trust new users yet, as they are visitors who just created an account, still learning the community norms and the way your forum works. Whether accidentally, or intentionally, new users can cause problems in your community.
We also want to hide any so-called "advanced" functionality from new users to make the UI less confusing for them and gently land them on the deeper UI as they get more experienced.
Thus, new users' abilities are restricted for safety -- both theirs and yours.
Users at trust level 0 cannot...
Send private messages to other users
Reply as new topic in the right gutter (UI removed)
Post more than 1 image
Post any attachments
Post more than 2 hyperlinks in a post
Have actual links in the 'about me' field of their profile (will be silently and temporarily converted to plain text)
Mention more than 2 users in a post