Why do you think that would happen? The entry will still have the normal "read" and "post" comment links, but they will link directly to the entry on LJ (or wherever). So if someone wants to reply to your entry, there isn't any extra clicking to do at all--it's exactly the same action as if they were reading it on LJ. Click "post comment", write comment, click submit. And all comments are in your original LJ entry. If nothing else, I'd think it'd actually encourage people who have already moved mostly or exclusively to DW to comment on their LJ friends' entries more than they do now.