The forum software has been upgraded and one of the new features changes the way you hide categories from the category view, http://forum.brickset.com/categories
, and the discussions view http://forum.brickset.com/discussions
Previously you would go to the category list, http://forum.brickset.com/categories
, click on the gear wheel on the right next to a category name, then select 'hide' to remove categories not of interest from view.
Now, you do the opposite: you click on gear wheel and select 'follow' for the topics that are of interest.
Once done you can switch between seeing everything and seeing just the content from the categories you've elected to follow by selecting 'All' or 'Following' from a new drop-down menu at the top of the page.
It does not appear to be possible to change the behaviour of the home page, http://forum.brickset.com/
, which shows all categories in category view.