In my forum users want to be able to see which posts they have read, and which they have not read yet. This includes posts that have received new comments since the last time the user logged in.
In the previous forum system that we used:
user would log in, all new posts would have a bold title, whereas the posts the user already read would have a regular style title.
In addition, posts that the user had already read, but that had received new comments since the last login - the user would see the title as bold.