I'm not sure if this is something I am fine to say, but I've never signed an NDA or something so I'll just say: (Moderators, feel free to hide this comment if you would rather not like an influx of alpha testers.)
If you sign up to the alpha-testing HN[0], you get access to the thread feature, which is... (quoted from my forwarded mail)
> New feature: when you visit a thread, the software now highlights
> comments that are new since you last viewed the page. These are
> displayed with an orange (or whatever your topcolor is) bar to the
> left of the new comment. Refreshing the page will reset these.
Very useful, I can't imagine having HN without these indicators any more.