Being able to distinguish new replies from existing replies in a comment thread is something that would be useful. On Reddit, you can get that if you're a mode

Being able to distinguish new replies from existing replies in a comment thread is something that would be useful. On Reddit, you can get that if you're a moderator or have a gold subscription. News and email clients would highlight new messages you just downloaded. What would be nice is if Hackernews would do something similar. For example, using the e tag header to decide which comments are rendered differently so that it's easy to see that they're new comments since the page was last viewed.