How can I get threads to show only posts that are NEW since the last time I looked at the thread?
it should do it automatically after you log in -- posts you've read are collapsed, with only the user name and date showing; new posts are expanded.
It won't work until you log in, though.
If it doesn't work like that for you, post a question on the Site Talk forum - someone should be able to help you.
The problem is, it only works once. If you inadvertently refresh the page, or if the page reloads itself (on certain hand-held devices with stingy RAM <grrrr>), the site will assume this is a new visit and the recent replies you were interested in will be buried somewhere in the discussion. It would be so nice in such cases to be able to sort the entire thread chronologically so that one could quickly locate the latest replies, but there do not seem to be any plans to provide this functionality:
An even better solution would be an option to redisplay the thread with replies added within the last x number of minutes/hours/days expanded (with the other replies collapsed).
I wanted to speak up in reply to your comment because there actually ARE plans to change the way users can sort the thread responses. (There is also a new look for both boards and threads that we are working on right now at CHOW.) This would allow users to look at threads in exactly the same way they do now as well as offer them the choice to reorganize threads by response time if they choose to. This is a very big project we're working on, and so everyone will be hearing a lot more about it in weeks to come, but I wanted to share this first mention of it because the part of your post involving chronological sorting is very much in keeping with what we're working on right now.