One thing I didn't see already pointed out is simply that the more deeply a particular node in a conversation is nested, the farther it ends up being indented over from the left side.. Thus, the more wasted space on the page.

After a certain amount of indenting (not much more than 5, I expect), you will have a very small column for the text to flow into (assuming you don't have your browser window set to be 1600 wide etc..). I expect this was a design decision as much as a technical / conceptual one.

- dynamo