I'm assuming here that many users will be using a bookmarklet/toolbar to vote a site up or down.
The idea is to have two links associated with your site, a positive link and a negative link.
When a user is on your site, you have an algorithm to determine whether his/her experience is likely positive or negative. If positive, you use the positive link. If negative, you switch him/her over to the negative link.
A happy user will likely vote up on the positive link. An unhappy user will likely vote down on the negative link. And so your positive link makes the front page.
Still you need some traffic in the first place.