I'm pretty sure you get downvoting powers at 500 karma. (At least, that's about when I got them. I have heard rumors that it fluctuates, and like all the mechanics of Hacker News, the precise rules are proprietary secret sauce.)
I have downvote in this thread but not always. I find the removal of edit, reply and voting privelege at seemingly arbitrary conditions to be very annoying.
Downvoting is disabled for very new comments and for replies to comments you've upvoted, iirc. Editing is disabled after a "reasonable" time. I don't think replying is ever disabled.
New comments have a brief time without a reply link - although you can still get to it by going to the comment itself through the "link" URL. The purpose is to slow down the back-and-forth arguments between two people that become "Nu-huh!" "Yeah-huh!"
Ha, and there we go, so I couldn't reply to you (no reply link) within the discussion but I can by following the "link" link and then replying. What's that all about?
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 ...
