If I made a blog where I post cool videos i find on youtube or other sites what kind of content would I put on a blog like that? How would I go about marketing it?
Well the content would be cool videos you found on youtube right? Maybe you would link to the video and write a little blurb on it saying why it's cool? To market it you could post links to you on the video page.
Content is more important, with video you need to add text content, just posting video is not going to help you.