While this service has good intentions and I do like it, I believe that in some instances the people rating sites lie in order to defame a competitor. And questionable sites rely on friends or associates to rate a site as good and reliable when maybe it itsn't. So the site rating is only as valid as the perception and honesty of the raters. I may be stating the obvious here but the more ratings a site has, the more likely the validity of the rating.
WOT is a good initiative in identifying malicious websites and suggesting genuine ratings by end users themselves based on their actual as well as factual experience.
Not always accurate, and definitely check out the ENTIRE scorecard not just the one little circle, there can be a number of red warnings about malware hidden behind a rather green circle in a search result :/ such is the nature of a community run tool though I assume, it is democracy...
Still, it's usually a good indicator. And it's an awesome concept. So I am very glad for it. Despite occasional annoyances :D