The disadvantages of viral videos.

Time constraint

Even though video is easy to capture viewer’s initial interest, it is not easy to maintain the viewer’s attention through its entirety. According to Nielsen Net Rating, people stay 17 minutes per session on the Website- in this short time, they sample several videos. Therefore, the video content has to be entertaining and captivating. This also brings about the constraint of length. Try to keep your video clips short, preferably less than 5 minutes long. Most people browse through a number of videos when visiting sites like YouTube and Google Video, won’t be willing to give videos too much of their time. Ensure that your video has interesting, entertaining or provocative content. If it doesn't engage your audience, they will have little incentive to share it with other people they know, or across their social networks.

It is not easy to balance between roughness and fancy

There are millions of short video clips on the web, produced by amateurs and professionals. The amateurs publish videos for fun and the professional publish their clips in order to drive traffic to their Websites and to sell more of their products or services. If it is too rough, it can be boring, but if it is too professional or fancy, people regard it as advertising. This is not something they want to share and forward with their friends.

The intention is not to produce a professional looking video that you’d expect from an advertising agency, instead in many instances, video clips are shot in an "amateurish" way and carries though a very ‘unprofessional’ image in their approach and production. This is done deliberately as a professional image would most definitely inhibit the viral potential of videos, and set the video s apart from the ‘ordinary’ videos created by individuals on the web. A point worth mentioning is the fact that although promotional, viral videos normally do not contain a sales message, product placement or a sales pitch on a landing page.

Obvious intention

When the intention of viral video is too obvious-to sell something, people tend to neglect the message. However, since we have specific purpose of viral video, it is not easy to balance between two. Also, the managing the linkage to the brand is one of the difficulties; This has to be subtle but also has to leave impression.
http://www.youtube.com/watch?v=RVeo-3lwwa8