The proccess by which a funny video, viral or not, is deemed unfunny by someone. This occurs when said person is told to watch a video by a freind, relative, the person who created the video, or some other person.
The reason it occurs is because the person being told to watch the vdeo is expecting something funny, which may not have been the case with the person who is telling the other person to watch the video (this person may have stumbled upon it, or if they created the video, overestimate how funny they really are)
Members of Youtube often destroy their chances of having their video go viral by saying things such as "omg go watch my video-it's so funny!" on the comments section of other viral videos-people are expecting something funny, and are let down when the video in question isn't as funny as they expected
Some videos are exempt from this effect, such as "HaHaHa" (the video of the laughing baby)
James told Shaniqua to go on Youtube so she could watch watch "Charlie Bit My Finger"- because of the Youtube Effect, James was on the floor laughing, while Shaniqua forced a laugh, expecting something funnier then what she saw
The phenomenon in which one navigates to a single video hosted on Youtube (either by URL from email, friend, forum post, by searching, or otherwise) and find yourself clicking on related videos until you wake up hours later to find yourself watching Chocolate Rain for the umpteenth time or enjoying the musical stylings of Rick Astley.
The YouTube Effect can apply to any other video, image, music, etc. site that shows related files or pages on whatever page you are viewing.
Shit, I told myself I'd only watch that ONE Lady Gaga video on YouTube, but before I knew it, it was 3 AM and I was watching mindless crap like fat ladies yelling at skater kids. Damn YouTube Effect...