Hi
"Perhaps I am missing something here, but how can the site know what posts are unread 'to you' unless you are logged in?"
You are quite right but the main point I was making is that, prior to a week or so ago, after logging in, new and unread posts were highlighted as were replies to old posts that you hadn't read. Now they aren't... even when you are logged in....e.g. Your reply didn't highlight this posting when I logged in just now, at least I didn't notice that it had..... I just noticed there were 2 replies. I think it's an option that needs to be turned back on as it was useful.
My comment re. the login button was an error on my part..In fact the "Please log in to reply" prompt is NOT an inactive button as I thought it was. The actual "Sign in" prompt is right at the top of the screen and as I had scrolled down slightly to see the top posting it had disappeared and only the "Please log in to reply" text was visible making me think it was a button. (Got so used to the full screen login page I forgot where the original sign in button was!) 
So..if you want to reply again I will check if it does highlight the post as it used to.... (after logging in.)
Nat