I really like the new delay-mark-as-read feature, since it allows me to quickly look for short posts and come back to longer posts later.
To make it work even better, I think it would make sense to mark a story as read when it is opened. This way, I could hit o to open a story in a background tab and k to go to the next new story, without having to hit u to mark the opened story as read. Implementation as a preference would be fine for me.
Hope this makes sense.