Sounds like a good idea at first, but I feel that not having collapsible comments is detrimental to the discussion, and in particular it counteracts the advantages of tree threaded discussion versus linear discussion (as seen on many messageboards and blog comments).

Why? Because the advantage of tree-based threads is that you can split off and have a relatively off-topic conversation without jacking the whole discussion. However, if such a discussion develops just below the top-voted comment, and it gets larger, it will dominate the whole discussion because people (naturally) won't always read the full page of the discussion.

This gets doubly problematic because of the tendency is that a "large, slightly off-topic discussion near the top comment" is very often about the same chewed-to-death topics that are just two camps firing off their default arguments at eachother (Apple vs general purpose computing, anything libertarian, intellectual property vs piracy, etc etc, you have seen them all). Which may be worthwhile discussions to have, nor would I want to prevent anyone from having them, but very often I want to read the discussion about the particular topic in this particular article, or at least how it differs from other articles triggering such responses, and not the general arguments by the two camps, which I already know (and most people have made up their minds about, anyway).