1) If a post is unpopular less people will be there to endorse.
2) With less people endorsing, replies will appear slower to people with low karma (lots of us), so we are disinclined to reply because we can't see the discussion. Less replies mean, less discussion.
3) The system also creates a strong disincentive to comment on unpopular posts because if no one reads it you can't comment on anything else. This will slow the comments of unpopular posts further.
One solution would be to weight the number of endorsements needed by the popularity of the post. The more people seeing it the more endorsements needed. For new posts (not popular yet) I would say comments shouldn't require any endorsement and let the existing down/up system rule.