Online forums are a great place for having a discussion (or an argument), and so useful for people who want to read those discussions regardless of their participation. Sub-forums and boards and threads are all very straightforward and easy to figure out. Easy to find stuff you want and ignore stuff you don't. I don't think forums are great for browsing art or blogging, but I think they are great for posting about games, or hobbies, or current events, or roleplaying together.