People have already mentioned some way of marking comments as read. If that turns out to be too hard to implement, here's a simpler idea: The ability to set a mark time that corresponds to marking as read everything before a given time. And then - rather than autocollapsing everything from before then, as this is a crude tool - add a recent threads tool, that shows all comments from after that time which are not a reply to another comments from after that time - allowing you to just jump to the highest-level comments you haven't seen and read the thread, rather than digging back through Recent Comments to find them.