I built a basic YouTube clone in Erlang that I currently only use in my home network. I've often wondered how much I could make if I came up with some flashy logo and name and tried to monetize it.
Things like building a scalable transcoding cluster can be made fairly simple ("just add more boxes") using a decent message-queue service. From there, everything is pretty simple HTTP.
Of course, this is only the very barebones stuff, and covering all of Youtube's features exhaustively would be quite difficult, and requires some good Machine Learning work.
The key to selling this though is to be able to customize to their needs, provide support and SLAs etc...
Hard work in all likelihood.