A bit more than a tweet, so you can express an idea fully, and a nudge, if you need more than a paragraph, you should be writing it on your blog not mine. After a certain size, you're not commenting anymore. You're writing a new blog post.
There are more features that would come after this. Like something to limit the rate at which new comments appear. Or a way to say "hold all comments until this time and then publish them all, and shut down comments." Think about how that would help keep people focused on the issue in the post instead of devolving into pointless permathreads and debates in the comments.
I've believed this since I started discuss.userland.com in 1998 as an adjunct to Scripting News. Every post would appear there as well as on the blog. And comments would develop underneath. It was from that environment that the first layer of the blogging community booted up. If you look through the archives, you'll see comments from many of the earliest bloggers, before they had blogs.
Here's an example of a day's worth of posts when viewed through the discussion group, on 7/14/2000.
And here's a blog post, with a comment thread below.