As cookingrobot says, gmails threading is one dimensional and super clunky when you have lots of replies. It auto collapses to far so you can't easily reply to a particular message in the thread.
Reddit has the best comment threading system of any site online. Quora is near the worst. HN is just slightly frustrating.
No one seems to get this, not HN, not Quora, Digg (though that doesn't matter any longer...)
Its about increasing the information consumption rate and navigation rate. Collapsing is critical to navigating threads with thousands of comments, even tens of comments!
Any UX designer who does not see this, is, in my book, worthless.
But when you open a message with lots of replies, there's no structure, and you can't reply to one person's comment (like I'm replying to your comment here). This would be messy in gmail.
As in Gmail?