Some of this (especially the new post indicators) can be provided by a Chrom{e,ium} extension I've been using called Hacker News Enhancement Suite [0].
I just installed this. How do I know when a comment is new? I just looked at a few threads i hadn't opened before and I was expecting all of them to have the "new" UI element but they all looked normal (other than the new UI that the plugin brings for all of HN)
edit: and as I post this, my own post is marked as new. For those interested, new posts seem to be denoted by an border on the left
edit: and as I post this, my own post is marked as new. For those interested, new posts seem to be denoted by an border on the left