Some people believe that they can add value to their site just by adding plugins to the page that disturbs the users. For example a plugin provided by snap.com which displays a snapshot of the link you are hovering. For me this is the most disturbing thing that happens when i’m browsing. I frequently hover links to see where they lead and when this little plugin pops up it disrupts entirely my experience. I am closing the site and leaving elsewhere.
You can see the screenshot i made on the Mantis page, but i am more offenced when it is on blogs. Because there you spend more time and you are more likely to get haunted by this little plugin. So if you use it on your blog or site, you will have a visitor less.