Something theoretically small, maybe just an icon if there is a new comment on a story you posted? Or at least a Last Comment: text box below the Created line or something?
I always wanted to add a feature that an author gets a mail for new comments. But surprisingly often, people are not logged on when submitting stories, which would defeat that feature.
Your suggestion is actually a good idea, but it wouldn’t help an author realize if he got a comment to an older story he doesn’t check regulary, at leat unless I put some more work into it (like a list of all stories of the author sorted by date of latest comments). Using Cookies for this would be the easiest solution, but I’m afraid many people put their browser into ‘stealth mode’ and disable all cookies this way…
My feeling about that is that if people disable cookies, they can’t expect features that rely on them to work correctly, so it’s up to them to decide if the feature is worth allowing cookies for or not.
Personally, I’d definitely use this feature if it were implemented, even if it used cookies.