@ Falseprophet - You are right that it can be manipulated. This is always the case with any user-based rating system. However, I will say that once a site gets large and hundreds and then thousands of votes are tallied there is a lot less gaming of the system that can be done.
Also the site admins work very hard to minimize flakey stuff, and also monitor for bots, etc. I still think it's a worthwhile plugin for Chrome, Friefox, et al., especially when a popup or errant click tries to take you to a know malware or spyware site. WOT will stop those from getting loaded every time.