One thing I could see useful is to get to comments for your current tab. Let's say you're reading something and you would like to see if there's any discussion around that on HN and if so, get to it. If not, get the option of submitting it.
I also remember that there was a bookmarklet that was adding links for each story to open a split window with the article on the left and the comments on the right. I liked it, though I lost it and never really looked for it.
I think what he's asking for (if I understand correctly) is a way for the plugin to check the page I'm currently on, say it's a mixergy blog post or something, and determine if that blog post has already been submitted to HN and if there are any comments about it. That would be sweet, that way I'd know right away without having to google search (site:news.ycombinator.com) for the url, and if there' already a thread I can join in the discussion quickly.
I would love to do something like that, but I don't think Hacker News provides any sort of API for accomplishing this. I'll look into it, though. Thanks!
Possibly better way to do it is to make a content script (or a greasemonkey script - same thing?) that rewrites links in HN's page such that when you open them in a new tab, you can keep track of the corresponding comments link, and then provide a Page Action that shows up in the location bar, clicking on which, will open up the comments page for the link.
This won't work where you directly happen to go to a site that was also on HN, but will work for the 80% use-case where you are going to a site from the HN front page itself.
This is not the best place to talk about that but:
As far as I know, HN doesn't have this capability indeed. However the script could scrape a certain number of the pages (it goes up to seven). You can easily match the linked URL with the current URL. I believe a Chrome plugin can have a list of remote sites it can do XmlHttpRequest on, no?
I also remember that there was a bookmarklet that was adding links for each story to open a split window with the article on the left and the comments on the right. I liked it, though I lost it and never really looked for it.