We don't have a "Who's online" list because we don't need to know who's online. Services like MSN messenger and Xbox Live show you an online/friends list because they are supposed to be quick use services, i.e I want a response/ game in the next 5 minutes or I'll do something else. The forums are more like a very open email service. Imagine the OP's post is an email sent to every user on the site. When someone replies, the email is again sent to everyone on the site, but with another comment attached to it. We don't need to know who's online when sending an email because it's not about the time taken for a reply but the discussion within. The other thing I don't like about this idea is the way it will highlight forum activity. It's not hard to figure out which forums are dead, but this feature would give reliable evidence on which forum has no users on it. Making spam attacks all the easier on the dead forums. But who exactly are you trolling in a dead forum? Nobody. Although, I'm not completely against the idea. :-)