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.
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.