It's a valid point... a filter by date range could help! Or how do you see it working?

For now, to help you partially on that... after you visit a post, its title will be colored in gray #969696 instead of white #f0f0f0 in the main feed list.

For the comment, after you visited a post and someone write in a thread you visited, a little blue icon will appear to alert you about new comment(s) under that thread. If you want to get notified, you need to subscribe to that specific post or that specific stacker (for any new comments or posts).