I was always kind of surprised that there's a relatively high karma threshold for downvoting, but seemingly not for upvoting. Seems like it would invite voting rings. I guess there's already other software to detect that though.
Downvotes per item are limited to -4 and the signal-to-noise ratio for negative votes beyond pushing them below the “new comment” boost is of little positive impact in filtering content, in my opinion.
And yes, there a lot of filters in place for upvotes, many of which are intentionally kept secret.
