YouTube's 'Related Videos' feature has a serious issue with privacy. According to Shumeet Baluja's paper entitled Video Suggestion and Discovery for YouTube: Taking Random Walks Through the View Graph [pdf], they're using a variant of the adsorption algorithm that is based on co-views statistics. That is: a bunch of people, who watches this video that you're watching, also watches the following videos that you might also enjoy, so here they are.
Here's the problem. Let's say I'm John Doe. I opened a YouTube account called johndoe and uploaded some home videos here. I also have an alterego named James Bond so I created a second account where I post work related videos there. I am careful to keep the two egos separate: I don't cross link videos from one account to another. But from my home computer I do occasionally review videos from both accounts together. All of a sudden, my James Bond videos show up on the 'Related Videos' list to my John Doe videos. And boom! The whole world knows that John Doe is James Bond, thanks to YouTube.
This is an extreme and fictional scenario, but the issue is very real.