Vine: A new way to share video
blog.twitter.com
blog.twitter.com
I think it's not any one big thing, it's several small things and details that have to align in just the right ways for adoption. For example, the sharing needs to be fast, the videos need to play right away without any action (like GIFs), there must be some fun component to it (usually in form of an arbitrary limitation that challenges creativity), the design must be clean and functional, and it needs to integrate well with existing services.
Vine has a checkmark next to all of those. The idea isn't particularly novel, but the execution is great and the Twitter integration helps. I particularly like the mechanic of recording only while a thumb is down. This might just be fun enough for me to use.