Down voting is too easy to abuse. Reddit was being gamed in this way -- certain users were down-voting everything, which made being a new poster starting out an almost automatic karma penalty of a few points. They've made the down-vote penalty milder because of this.
I suppose, but if you only get one...you'd have to have an awful lot of extra accounts to make a big difference. But, if everyone agrees that a story sucks or doesn't fit the purpose of the site, it ought to go away.
Just having a small amount of extra accounts can make a big difference if you can down-vote stories at the very beginning. This gives the unmolested stories a big head-start. I think that reddit implemented different behavior for the first hour of a story's life for this reason.
Only one down-vote a week? If you reduce the frequency like that, then that's just another way of weakening down-voting, which is what reddit seemingly did to their downvotes.
