If I understand you correctly, you want to have a comment box appear when you click into a thread from a discussion item. I have to assume you're using the new thread view, with the reply button, etc.
I don't do this for two reasons:
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There's some bullshit that the comment box code pulls that makes it difficult to automatically load, and to do so while letting you have the ability to change the comment box to reply to another post entirely. I remember fighting with it when I first put together the new thread display mechanism, but I don't remember anything more specific.
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I don't want people confused on what they're replying to. In classic thread display mode, you're only ever displaying 1 post at a time, and so which post you're replying to is absolutely obvious. But despite this, even in classic mode, I find that one of the common complaints people have, or mistakes that they make, is that they reply to the wrong post. By forcing users to click on the post they want to reply to, and seeing the comment box immediately beneath it, this helps make which post you're replying to incredibly obvious. It means you have to click twice, but you'll never be confused on what you're replying to in the sea of text that is the new thread display mechanism.
In all honesty, I'm trying to avoid a terrible mistake I made in interface design on v2. If you remember, on v2 you had the ability to reply to a thread directly from the discussion item view. You would click the "reply" link, and the post box at the bottom of the page would update its hidden variables to say "I want to post to this thread's post, not to the discussion item itself." However, the only thing that indicated you were doing anything different were those hidden variables. Until you hit "submit," you had no idea if you were posting to a new thread, or posting a reply. And so people mis-replied all the time, and got frustrated because they couldn't fix their mistake, when really it was just a result of poor design on my part.
We lost a lot of good users back then.
