One thing I noticed from K5 is that having both options leads to lobbying and information cascades. That is, if one or two early voters post comments saying the article is really good or really bad then that hugely affects whether later voters hit +1 or -1. With only the option to vote up, people might point out flaws in the article or praise it, but the comments are more intellectual and less designed to influence the voting of others. Of course K5 is much worse because the design strongly encourages people to vote one way or another, whereas on this site the bad articles will quickly disappear if people just ignore them.