While I realize that that PunBB probably has a lot going for it and I assume that you made an informed choice when you picked that as the forum software, I want to add my two cents on the topic anyway.
The best forum software keeps track of exactly which posts I've read, and which I haven't. Even between sessions. That's a feature I find very useful, since it allows me to log in and read just a couple of threads if I don't have much time, and the forum will keep track of which threads I have read. It's a very nice timesaving feature. Those forums usually also have "Go to first new post" buttons next to each thread, which come in handy, particularly in longer threads.
It does use a bit more database space, since it requires an additional table to keep track of what each user has read, but it is the single most useful feature of any forum I've visited. I haven't looked at the source for PunBB, so I don't know how easy it'd be to add, but if it's easily done or if you're in the future considering switching forum software, that's a feature I'd love to have. It'd probably make me a forum regular. (Oh no! I shouldn't have said that. Now I'll never get that feature. )