I mean, this looks minor to most people, but from a usability perspective it is big. It means that LJ is actually tracking when you last read a thread, and which comments came in since then. And *that*, in turn, means that it's a very small step to implement "Show me all of the flist entries *that are actively being discussed*." It allows you to do client-side stuff to de-emphasize the bits I've already read, so that I can quickly focus on the new material. It's basically the poor man's version of a key feature of CommYou, the ability to track where the interesting conversations are happening.
I wonder if they have any clue how important this one is, or what to do with it? (And now I wonder whether DW already has this feature -- I haven't been paying much attention over there...)