Something I have been considering for a couple weeks is traffic. What if inbound external links were sandboxed until the page on which they reside is viewed x number of times or the link is clicked x number of times. That way Google doesn't have to care whether a link is reciprocated or paid while assigning its value. Links on the millions of link pages that noone visits would not count as votes. A true linkvote would be one where there is a reasonable chance someone may click it and see where it goes. After all, that was the original intention of PR.