There's a new post on the ReelSeo site that gives some good advice about video marketing on YouTube.
What they are saying is that the description box can be a rich source of SEO goodness if used properly. Using well thought-out copy and including relevant links will bring benefits to your video marketing effort. It seems that YouTube decides how and where to rank your video not just based on the tags you assign, but also according to keywords and phrases used in the description. Not just YouTube, but other search engines also crawl the info placed in the description box. Getting your video seen by as many people as possible is the idea here, so anything that helps with that is good...
So what you want to do is carefully plan your description while uploading your video, with regard to some basic suggestions:
The first 5 lines of the description are the most important - they sit above the 'read more' fold, so are always seen by viewers. The idea is to have a strong traffic-driver in this section along with at least one link to your page. Also, the first 2 lines are displayed next to the video in search results and in social media.
Plan and use keywords and phrases that really pinpoint your product/service/subject/niche.
Use links (type in the full url including 'http://') in your copy, preferably in the first 2-5 lines. Make these links relevant to the video subject and destination pages.
There is a 500 character limit, you can go to town in your description! If you are promoting a product or service you can write an in-depth summary of your topic (or even a transcript) that will perk up Google's antennas and hopefully drive more traffic to your site.
There's more detail on the ReelSeo site, as well as other info related to all things video-marketing... Read the full article on ReelSeo here