My favorite feature is the New Post alert. I use a PC at work, and a different PC at home, but whenever I go to this chat forum, the forum knows which posts I've read and which I haven't.
The ones I haven't looked at yet get an orange 'New Post' next to them, whether I'm at home or work. And I never have to log in
.There's other reasons too, like Watch This Post (alerts me when someone answers) and the ability to start and vote in Polls.