I think this is a very serious possible problem with this sort of system, but there are ways around it. The system could take everyone's whitelist and derive a

I think this is a very serious possible problem with this sort of system, but there are ways around it. The system could take everyone's whitelist and derive a "public whitelist" that it can use to filter articles and comments for people that are new to the site and have unknown preferences. This "public whitelist" could be derived from an average of everyone's whitelist, with other heuristics thrown in. As the system learns what type of posts a new user prefers their whitelist is updated. One can also imagine how a user could use a whitelist on their own posts to maintain a separation between the things they would say in public vs. their private thoughts (which others may find offensive or unpopular)