Welcome back, qnorth! The new forum takes some getting used to, but It works ok. Categories don't separate easily since new posts re always on top, but you can find the original categories if you search.
When you block a person, they can no longer invite you to a private message or post to your profile wall. Replies and comments they make will be collapsed/hidden by default. Finally, you'll never receive email notifications about content they create or likes they designate for your content.
Note: if you proceed, you will no longer be following .