How does one get down-vote?
You don't downvote submissions. You just don't upvote if you don't like it.
Downvotes are disabled for any comment more than 24 hours old, but upvotes are still allowed after more than a day. Emphasize the positive.
My main point would have to be that you have to work all of this out. Shouldn't it just be written in the FAQs. You could make the UI more helpful, like keep the reply link but disable it (eg with strike through) or have it popup a note "new comment: reply disabled for 5 minutes" or whatever. Or on pages with no downvote have a header comment in a box saying "downvoting disabled; all stories have down-voting disabled after 24 hours".
Gah, I have to stop before I get on to not having collapsible threads and the lameness of greying out comments and not having vote count displayed and ...