You need 500 karma. The limit is increased every once in awhile, but this seems to still be the threshold since I just crossed it last week.
Interestingly, after I hit 500 the downvote option didn't appear so I wondered if the threshold might have changed, but it fortunately became available later that day.
Edit: ceol is right. That matches my experience, too (verified by looking at older posts).
You need 500 karma. The limit is increased every once in awhile, but this seems to still be the threshold since I just crossed it last week.
Interestingly, after I hit 500 the downvote option didn't appear so I wondered if the threshold might have changed, but it fortunately became available later that day.
Edit: ceol is right. That matches my experience, too (verified by looking at older posts).
From what I've noticed, you only get it for the comments on articles since you passed the threshold. You're unable to downvote old comments.
there are more features at higher karma levels, too. nobody really talks about it, though.
I can't downvote yet, but upvoting something because I was trying to scroll irks me.